I recommend you ask a few racers at the track you plan to attend for the gear ratio they use to get a ballpark idea (wing/non-wing, length of straights/tightness of curves, slick or tacky all make a difference). For example, we may run a 12/52 in 3rd in our R1 on a 1/4 mil track for a 11.54 ratio. You will need to look at a gear chart for the GSXR, as the primary ratio and the individual gears will be different by motor. Check out
http://speedpartz.com/techarticles.htm for the chart applicable to your '08 motor. Looks like a 12/52 is pretty close for your motor too (11.53) or a 13/47 (also 11.53) but if you prefer 2nd gear you could go with a 12/43 (11.42). Use the tach to adjust from there to get into your target RPM range. For a 3/8 mile track you may need to be under 11.00, for example a 12/49 in third (10.87) or 13/45 in second (11.04)
The other consideration is chain length - depending on the other tracks you plan to race, it may be better to go with 3rd than make up a short chain for the smaller gear (assuming you run out of adjustability in your radius rods). I agree with jjones - use a tensioner; we've used Speedparts for several years, replacing the blocks periodically, and have had no issues.