The above charts make me wonder if its really the second cat that is the restriction point? I know cats are supposed to flow very very well but its weird you picked up such a significant gain by simply removing the second cat with no tune. Speedlab was able to eek out about 5 hp more after tuning (on top of their header). I bet with a tune and some higher octane gas you would be in the upper 80's for PS/HP which according to your dyno would be about a 25% increases over stock.
A 25 percent increase in HP with basically headers, a tune, and a second cat delete, would be impressive in this day.