My best growth happens in the upper 70s to upper 80s. Low 90s seem good too if humid and a bit of wind, I would fear fungal problems are mid 90s in a humid low wind greenhouse. Also, night temps can't be too high or you won't get fruit set.

You could try fabric row cover instead of plastic over the low tunnels to prevent overheating.

I think your results will vary over time because it isn't just air temperatures that influence growth but also soil temperatures, humidity, amount of hours of sunlight per day, nutrient availability in the soil, etc.

In our typical early March weather, I think the greenhouse tomato plants will grow more rapidly, but they will need a lot of wind exposure (I use a giant fan on wheels that is easy to move around) to help them develop thick stems. By April, when it is getting really warm (for me down here in far southern OK, up there where you live, maybe the real warmth arrives later in April or even in early May?), the growth of my in-ground plants will outpace anything left in the greenhouse. The greenhouse plants might get taller since they stay warmer, but they won't spread out as wide (since they are in plastic cups in plant flats) and the in-ground plants will quickly spread out broader and wider. My greenhouse plants are under 50% shadecloth to keep them cooler and that probably slows down their growth some too.

Temperature and humidity work hand in hand with tomato plant growth. I see the best growth in April and May when daytime highs are in the 80s and nighttime lows in the 60s. I get the best fruit set during the same time frame and then fruit set begins to slow down when we start regularly hitting the 90s. It slows down earlier if we are very hot and humid simultaneously. Since my weather tends to be hotter and drier in drought years, I won't have quite as much trouble with humidity impeding fruit set in those years, but in wetter and more humid years like we're having this year so far, the high humidity is more of a problem.

Particularly in May (it could happen in April this year since we've been so hot so early and already are hitting the upper 80s and lower 90s some days), you can see your tomato plants that are growing in the ground grow an inch (or more) taller per day when they are getting exactly the weather they like. Sometimes, if we don't get too hot too early in June, that rapid growth will continue for most of June. It slows down, though, once the plants are getting the hotter temperatures and also because by then a lot of their energy is going into enlarging and ripening fruit. In especially hot years, like 2011, we were already having hot enough air temperatures to cause blossom drop in mid-May, and even on occasional days in early May so it isn't all about plant growth---you want to create the conditions that get you fruit set as well and too much heat can impede fruit set.

With most tomato varieties that set large (as in anything bigger than bite-sized) fruit, temperatures in the 92-96 degree range tend to start causing the blossoms to fall without forming fruit, so you want to carefully monitor temperatures inside both your greenhouse and low tunnels to avoid blossom drop. You also would see poor stunted growth if you have tomatoes in the ground when soil temperatures are below 50 degrees at night even if the tunnel is keeping them toasty warm during the day. Usually, though, the increased heat during the day will warm up the soil in the tunnel area enough to prevent stunted growth unless your nighttime lows still are cold enough that the soil doesn't hold its heat well enough, even in the tunnels, after sunset.

I use floating row cover over low tunnels when cold temperatures or severe weather with hail threatens, but don't use plastic. The row cover helps hold in warmth and reduces the hail threat to almost nothing, but I don't keep them under it once the temperatures warm up because it reduces the amount of light they get and increases their temperature to the point that they can get too hot, although that is more of an issue with the row covers that are a heavier weight than with the really light floating ones. I have four different weights of row cover, so I can use increasingly lighter weight ones as we heat up. So, with plastic, you'll have to monitor temperatures inside the tunnels. Even with open tunnel ends, air flow may be impeded and that could lead to the development of foliar diseases so you'll have to watch for that as well.

Back before I started using floating row covers, I would wrap my tomato cages with 2 to 4 mm plastic as soon as I put the plants in the ground. I left the top open for good air flow, and I removed the plastic whenever the plants had spread out wide enough that their foliage was beginning to touch the plastic. In effect, each caged tomato was inside its own little mini-greenhouse, albeit one with no roof. Most years, I only kept the plastic on the cages for about a month to six weeks. It wasn't a perfect system because the plastic cut wind movement, which kept the plants' main stems from getting as thick and wide as they would have with more wind exposure. If cold weather threatened, all I had to do was add a layer of plastic over the top of the row of cages to keep out frost. I also didn't like leaving the plastic on once the plants were blooming since they need the wind to 'shake' the tomatoes' self-pollinating/self-fertilizing flowers to make the pollen move around properly inside the flower in order to get fruit set. You can get around that somewhat with plants under plastic by gently shaking the plants or thumping the flowers. I just had too many tomato plants growing to spend a lot of time shaking individual plants or thumping individual flowers. After all, while the plants were growing in their plastic-wrapped cages, the rest of the garden was demanding attention as well, so I couldn't spend all my time only on the tomato plants.

I would think low tunnels would work great early in the season when the wind is strong, like it generally is in early March and some of April, provided you have good enough air flow to prevent high humidity in the tunnels from contributing to the development of diseases. The hotter it gets, though, the more likely it is your plants may overheat so watch out for that. I'd put a thermometer right inside a low tunnel and modify air temperatures that way.

You also may see more spider mites on plants under low tunnels because they'll like that increased heat.

One way to test the effectiveness would be to have the same varieties growing in three ways at the same time: inside the greenhouse, in the ground under plastic-covered low tunnels, and in the ground without row covers. Then you could compare how all three types grow and produce to see which method works best for you.

You also could grow them under modified tunnels where the plastic serves as side walls to help hold in heat and help moderate the strength of the wind, but with the top completely open to allow more wind flow and to prevent overheating.