I think at this point your limit is going to be the green injectors.

If the motec m84 can run high impedance injectors you're better off going for modern Bosch EV14 injectors (600cc maybe? - a retailer will be able to help you here!)

IF it cant Greys / Light Blues / Black Siemens injectors (Im not too clued up on these and their power, i think greys will do 380bhp, blues 400-420? Have a search on here - again though i would much prefer modern EV14's over any of these more old school injectors)

Once that's done your next limit is the hot side of your turbo, whether its a 0.48, 0.55 or 0.64. Generally from whats on here a 0.64 will see a genuine 380bhp.

Past 380bhp you'll want to start looking at a bigger turbo, GT30 etc etc