like others have said, you'd be hard pressed to improve upon the single piston calipers without spending a sizeable chunk of dough in both the brakes themselves and corresponding rims to fit and rubber to harness the newfound stopping capabilities.
i too would look toward better pads, a master cylinder upgrade and probably a proportioning valve to dial it in. some stainless steel drop lines while not a huge game changer will take up a bit of the mush in the system from rubber lines, so that's worth considering as well.
