I meant to respond to this post earlier. Here's my two cents:
1) If I usually print @ 8x10 or less with the rare 16x20 will I will be able to detect a difference in IQ?
No, definitely not at 8x10 and if properly framed, exposed and processed, not at 16 x 20 either
2) Since I usually don’t go above ISO 800 nowadays given the XT’s limitations – whether ISO 6400 is plenty good for school plays/recitals? it seems from some threads that the Mark III is doing ok 12,800 or even 25,600 granted I can’t tell how well they print.
Harder to answer. Up to 800, not a problem. Above that, you will start seeing some differences. Remember though that in order to get the benefits of the higher ISO speeds, you will need to retain the same lens f-stop. You already own an f 2.8 lens for wide to normal range. You'll have to add in the cost of a 24-70 2.8 to get a comparable range, and remember that your lens has IS, so factor that in as well.
Also, remember that with your telephoto you will be giving up 1/3 of the range if you switch to full frame. To achieve the equivalent of your 300mm lens on your Rebel or on a 7D you'd need 480mm on a full frame.
Now, here's the standard answer: It all depends on your priorities, needs, wants and budget. Only you can answer that. What would I do? First, I'd keep the Rebel XT through the summer and see what happens in Sept. with the 7DII so that I'm comparing the 5DIII with the same generation of 7D.
Second, I'd ask myself if the difference in cost might be better used in giving the kids a memory they'll have for the rest of their lives. Between the camera and lens upgrades, you're talking two to three thousand dollars difference. I'd probably take the money and take the kids on a trip they'll never forget. Fifty years from now, they won't have any idea what camera you shot those vacation pictures with, but they will look at the pictures and remember standing by your side watching Old Faithful erupt.