Yes, absolutely! My car is completely stock other than the Cortex and it performs like a completely different G8 now, thanks to the tune. The transmission tune alone is worth it, but I definitely can feel more power as well. The car just continues to pull in the upper RPMs in every gear where it didn't before.
From what I've read, the cortex plus Rotofab (and the Rotofab-specific tune) should net you around 20-25 rwhp, which is absolutely noticeable.
2009 G8 GT, Liquid Red, Roto-fab CAI, Cortex tuned.