Dont think there are any bigger master cylinders you cant fit to be honest, i just bought a pair of standard 2wd discs for £60 as i cant see the drilled/grooved ones making much difference, i think a better quality pad is more important. dont know about the rervoir does a 3i one fit? if not just block the 2 pipes up, saves swapping it

edited cause i forgot, speak to zoo about removing the ABS as brom did my old S2 and was loads better after