My question is, if I had the 17-50 Tamron f2.8 nonVC for indoor use how much would it of improved my ISO or pictures?Well, the difference between f/2,8 and f/4 is one stop. So, if you're shooting 3200 ISO and f/3.5 and f/4, then you'd be shooting at ISO1600 with the Tamron. Maybe ISO800 on shots you took at f/5.6.
Here are some examples:
1. Some of the really dark pictures from the rides I was at f3.5 at 3200 ISO and they were still not great. What the f2.8 of been able to capture the shot?
2. If I had indoor pictures at 3200 ISO from f4 to f5 then with the f2.8 what ISO would I get and would I be able to avoid using a flash?
Some things to consider; if you are taking more posed shots (not kids running around, etc) then IS would allow you to lower your shutter speed by 3 stops or so. But, if its for moving subjects, thats not help. Likewise, an f/1.4 lens would represent a 3 stop improvement over your current lens, and f/1.8 is a little more than 2. Those are big enough differences that you'll either get faster shutter speeds (less blurry shots) or be able to shoot lower ISO.
If you like your 15-85's range, then a prime or two in that range is probably your answer. The 50mm f/1.8 is cheap, the f/1.4 version would be even better. As someone mentioned, the 28mm f/1,8 would be good for shots that are a little wider, and the 85mm f/1,8 would be good for closer in shots. I'd see what focal length you use the most and grab that first.
As for using them on a body that doesn't AFMA, then you want to either be able to test it out before hand or buy it from somewhere that allows easy returns. Or, send it in to Canon to calibrate.