Unfortunately the dyno plot which I used to make as my point of reference for the difference between a stock cat when clogged and a new high-flow unit was on g-speed.com which has been taken down. The car put down a number in the 140s with the old stock cat, and then with a high-flow unit it put down 155 hp. THIS IS BECAUSE THE STOCK CAT WAS CLOGGED.

There are a bazillion dyno graphs out there of GSRs with stock cats and i/h/e, here are a few examples. Variances of +/- 5 whp from car to car aren't really indicative of any trend.