I recently upgraded from a 2 Bros slip on with Flies out, and PCV to the Area-P full system & a BMC filter. The difference is significant above 6,000 RPM. From 6k up to 10k, the bike pulls so much stronger. I didn't do a Dyno pull with just the slip on, but it pulled 147.7 hp with the full system installed. My mpg also increased from 43 to 48.
Several people have commented that Dyno numbers can vary between Dynos, and different days/temp/humidity. This is very true, but check this out; 2 dudes dropped their Hayabusas on the same Dyno right after me, and they pulled 155 HP and 163 HP. Both of them had aftermarket pipes and who knows what else.
This thing sounds totally Sick!