Here is my opinion/help.
For performance I highly suggest getting the JB4, BMS downpipes, BMS intake and meth kit, helix IC and an aftermarket chargepipe. Terry is a stand up guy and will always try to help you, Terry is the owner of BMS (burger motorsports). Their forums are n54tech.com and they are more performance based than these forums.
Some guy put down 385whp and 479rwtq with similar mods. You probably won't get much more out of the n55 yet. The n55 is still new so im sure they will eventually get over 400rwhp.
I myself came from a 500rwph+ fd rx7 and the power itself was a downgrade but I love my car. If I had 20k to spend on mods I would love it even more.
There is no blow off valve upgrade for the n55 but I have heard the intake has the same effect and there is videos to prove how loud it makes the diverter valve sound.
I personally would go for a C63 instead of a 135i though
