We recently installed a Corsa catback and "X" style crossover pipe on our 2009 G8 GXP. We put the car on the dyno to get results from just the exhaust, the exhaust and a tune, then the exhaust, Rotofab CAI, and tune.
For the small amount of mods, we are happy with the results. Now we just need the weather to stay nice so the track can open.
Stock Baseline 371 RWHP / 382 RWTQ - After Corsa 383 RWHP / 384 RWTQ
Corsa Exhaust & Livernois Tune 396 RWHP / 401 RWTQ
Corsa Exhaust, Rotofab CAI, & Livernois Tune 410 RWHP / 410 RWTQ
Here is a video of the car at start-up and idle, cruising at 30 mph, 70 mph, the 1st-4th gear WOT. (excuse my slow shifts )
and a video of it on the dyno with the Corsa Exhaust