holy moly thats a lot of incorrect information
basically the engine stops making power at around 8200 rpms.
when anyone finds a gts N/A that makes power to 9K rpms let me know
the next question,
why shift at 8600.
simple answer =lift
complex answer= in order to get the best possible acceleration you want to stay on the highest curve in the powerpand, on our cars with tuning on the pfc we can make peak power at 8200 rpms, but only loose about 5whp by 8600 rpms, if one were to shift at 8200 rpms (and still land in lift) you would be making less horse power at 6000-6500 rpms than if you held the gear till 8600 and then shifted and landed around 6600 rpms.
at 8600 rpms on our cars the engine makes the same power at 6600 rpms therefore that is the idea "sweat spot" you want to stay in for maximum acceleration.
gearing and tire size play a role in the shift point also, you do not want to shift 4th at 8600 rpms because 4th gear is so (numerically shorter) than 3rd gear, when you up shift 4th gear at 8600 rpms you rpms drop to about 68-6900 rpms, this would hinder acceleration because the sweet spot is right at 6600, so you would need to upshift 4th gear around 8200 rpms in order to stay in the engines sweet spot.
this is all with the apexi power FC of course.
btw tell your friend hes a tool, and redline doesn't mean the engine stops making power after that point, manufacturers put that there to stop the average idiot from blowing his engine(which is also why rev limiters are installed on production vehicles)