Chubbs, someone said before that the reason you're loosing up the top end would be cause of wastegate (doubt that as it's brand new) or the Bov... the standard GTR Bovs are a great little upgrade for our cars and can hold a bit more pressure before releasing any air... and, although it's heresay and some say it's a straight out lie, our bovs are designed to leak just a little bit as standard... some say for smoother gear changes, I say I have no idea why
Might be worth looking into though
Man, your car just keeps getting better and better!