imo GT-RS + 3L will be rubbish and be falling over at 5k rpm without making anymore power than what the 25 can do
Disco, whack a good intercooler on and up the boost with the 98 tune, if you are concerned about legalities than get a Blitz turn flow thing, factory intercoolers are rubbish unless its off a Rb26...hell my JZA70 intercooler is 3x (if not more) the size of a 34 side mount and its holding me back at near stock power
or for even less money than E85 there is WMI