I have not dynoed my own bike yet, but here are Yoshimura charts comparing the stock GSX-R750 vs. Slipon vs. Full system replacement. The top end changes are as one might predict. Removing the exhaust valve changes the low end. I think the graphs are best used for comparison, as peak numbers can vary for each dyno.
An article with dyno results compared the 2011 GSX-R750 to the 600 with the 750 having 21 more HP and 10 ft-lbs more torque. Sound tests showed that the 600 as louder than the 750 for some reason. The 750 had less exhaust noise but more intake noise.