A two stage air handler will do absolutely nothing to improve cooling to the 2nd floor of your home in the summer (except pad the contractor's wallet). Get a separate system for upstairs if you're having problems. Any standard, ducted split system will work. Don't know which ducted Mitsubishi system he is proposing but they tend to be on the expensive side.

@skt77a - appreciate the response. So far, i've head several techs, entirely independently of each other (different firms), suggest that a two stage variable will make a noticeable improvement (although would not eliminate the problem completely). Curious - if that's wrong, is there a common misconception about the relative effectiveness of two stage variable? Tx

Because heat rises, it's always a challenge for one system to keep multiple above ground floors cool in hot weather. Putting in a dedicated system for the upper floor will most certainly resolve the "too hot in summer" problem. If you do that, you can probably use a smaller size when the existing 15 year old equipment is replaced.

I think the point of multiple stage equipment is for the "conditioning" to be more gentle, not a blast of hot (or cold) air when on then nothing when off. There's instead a lower stage that continues at a lower speed. In humid summer areas, that helps with keeping humidity down. But not with coverage. If there's not enough return capacity and supply ducts on the upper floors with the existing system, operating at a lower fan speed will make the situation worse, not better.

A 4000 sq foot multiple level house with a single 5 ton AC is guaranteed to have temperature variations. A variable speed AC will not fix this. You need either install first and second floor zones or a second floor system.

I am not a fan of installing an AC system in the attic but it may be the most practical solution given the construction of your house.

I guess I would say that the only way a 2-stage variable speed system would work is if it paired with a zoning control system.

I guess I would say that the only way a 2-stage variable speed system would work is it is properly designed and installed AND if it paired with a zoning control system <That is properly installed and designed.

What is 'properly'? The definition of whomever you choose is likely not the same.

1 stage, 2 stages, 5 stages paired to one control will never make up the difference to a system 'with more control' of the system. The more control of a system you have the more it will cost upfront. If you don't approach it from elegance it probably won't amount to much after about 5.5 years, maybe less. (I work on junk zone systems every summer.) 25% of this HVAC job is knowing what to do, the other 75% of this job is knowing WHAT NOT TO DO.

If a thermostat is on lower level it doesn't know how hot or cool it is on upper level.

If a thermostat is on upper level it doesn't know how hot or cool it is on lower level.

For HVAC to work: it has to work in 80 degree weather and 50 degree weather, just as much as 100+ degree weather and 0 degree weather.

I service the Katy, Texas area.