JP4 sucks on a vspec
Try your clutch set up. I run 2 black and 2 green, OR 2 green and 2 gold depending on the track.
I have been running vspecs for 3 years now, I do not run ANY JP pipes as they all suck fuel AND they dont run on a nice crisp power band.
I run the RE11 pipe, but my fav is the dyn 053. Im not a super pro that knows it all, but I have been running the same setup for a while, I have played with setups, pipes, fuel, ect.
I dont think you need to spend that much on a pipe. Either the Losi Re10 or 11 would do just fine.