Three of which are in an upstairs gameroom that gets incredible heat from windows. It is very windy here and we get lots of wind noise and probably wind entering thru the builder grade windows
Low-E366 is your best choice, and choosing a window with low Air Leakage rating (.08 or lower) is very important as well. While the Simonton 5500 and NT windows are comparable in quality (both being mid-range), they both have higher AL ratings.
And they bid to temper glass in bottom 3 great room windows as this is apparently code bec of window size less than 18" from the floor. Some companies say it is the homeowners "choice".
This only applies if the glass is BOTH within 18" of the floor and over 9 sq. ft. of solid glass. If you're putting in double hungs this most likely doesn't apply and you should save your money by going with standard annealed glass. If you're putting in picture windows, they very well may have to be tempered.
First - do I want / need the 366 glass? Pros and cons - will I like it from a view standpoint?
Yes, 366 is an incredibly good product for Texas, and it's not overly dark or green.
Would foam in greatroom windows make a significant difference?
Foam filling doesn't make a significant difference generally, particularly with NT, since it's merely an insert.
Neither company shared the rating on air transfer - what is this one called? I'd like to do the best we can on this rating as we have tremendous wind noise now.
They may not even now what an AL rating is. The vast majority of contractors out there know very little about window technology, testing, etc.... Both the Simonton 5500 and the NT Traditions are going to have AL ratings that range from .12 - .18. These are very average ratings, certainly not in the premium window category. Sunrise and Soft-lite are both available in DFW and both offer AL ratings in the .01 - .04 range that will serve you much better.