If you adapt 250mm S13 discs and calipers to your brakes, (and get some good pads) your car will stop as good as it goes.
I run the stock dual resivoir 11/16 B110 M/C on my 1200, with the S13 fronts, running the sh1test pads money car buy. (Silverline from repco)And it stops so well, it's hard to imagine how it could stop better, even with better pads.
If you've adapted 120 discs to your car, then it shouldn't be too hard to make S13's work.