U can run P3.5 with the standard turbo, but your pushing the turbo to it's limits - so unless it's a recon (which ain't that good anyway) the turbo is gonna start running out of puff !! and you'll need a new one.
Whereas the K26 turbo will take that sort od boost with no problem. If your lookin for reliability then get the new K26 turbo - about £1500 from wallace.
Or for £550 or there abouts buy a nitrous kit and run 250bhp off gas and add 100bhp of gas = 350 bhp !! obviuosly to get that figure there are a few othr bits and bobs to get but won't cost near £1500 !!!