As far as i was aware RB26 injectors are top feed and RB25 injectors are side feed....so thats a no no to that plan Adam...but you can definately fit a GTR fuel pump...ummm i wont change the plenum as it flows well for above 300+rwkw so why bother changing what doesnt need to be changed....get your cooler, exhaust, give it a good servicing with irridium plugs regapped, and then you can crank it up to 12 psi....and you will definately feel the difference then....
good luck with it mate