How much depends on how risky you want to be, I've seen these run 2.5 bar before now, around 37psi, the greys will at the std rail pressure run 450 no problem so leave them in, saves buying some more, and leave a little extra room for further mods.
Your T34 will be the limiting factor, a T34.48 for example will run around the 360/370 mark flat out, with a larger turbine .63 (exhaust housing) will run more, I'd say around 390bhp.
Your head will also be a limiting factor as they are pretty shit to start with, a decent £300 port job will pay massively in the drive ability dept when set up.
If you're serious about your car, then I'd seriously consider moving away from chips and go after market, best thing I ever did, especially for MPG.
I did Watton, to Scarborough on one tank in my cossie at average 44mph, going A17/A15 route so no motorway work to speak of and averaged 40.8 MPG, with close on 200, 000 miles on the original bottom end(head was already done).
Having just rebuilt the engine it is being mapped(in watton where I live) with a lot more boost on 16 may, just got to put my rad back on after modification, fitment of a rail on top where the oil cooler sits.
tabetha