I was going to post the same thing, but yes if you already have S2 calipers and not bought any 4x4 ones, I'd spend the £40 on the bracket set you need and use S2 calipers with 2WD discs, a) cos of the 5% more efficiency, and also you will retain your pedal feel as the calipers are matched to the servo (alledgedly, swapping to 4x4 calipers could result in a softer pedal feel)

Then you can also afford to spend another £40 on some decent braided lines too - muchos better than poxy rubber ones So spend £80 on brackets and lines, or spend £100+ on 4x4 calipers... Up to you, but both set ups are a good improvement on std set ups