I got grade #1.5 roses when Hortico ran out of grade #1, and they have done very well. They were grafted on multiflora.

These were smaller, but healthy.

I was out working in the large rose pots yesterday, weeding and deadheading, and I ran across the original plastic label from one rose. I always put those with the plastic label I make staked through the hole in them. I was surprised to see that it was labeled as grade #1.5. I'm pretty sure that it was a Star rose, but for the life of me, I can't remember who it was. I ran across another label from the same company that was grade #1. The #1.5 was just as big as all the rest of my roses, but it's been there for awhile. I wouldn't call grade #1.5 "jumbo", since it's a grade down from grade #1. Grade #2 is another step down from grade #1.5.

I purchased ’jumbo’ and ’specimen’ from SpringHill, both meant to be larger than #1. I thought the size differential didn’t justify price hike. I’d just go with #1 going forward.

Of course you’d want as big a plant as possible, but I’ve also taken some 1.5 and they grew fine. My top objective is to get the variety I want, size and rootstock are very much secondary considerations

Ken, after reading Ben's comment above, I realize that I misunderstood your original question. I thought you were calling #1.5 roses jumbo. I didn't realize that SpringHill offered "jumbo" as a choice. Your question makes a lot more sense now, since I know you are a very experienced rosarian... apologies!