From what I hear, a good upgrade that is a direct bolt-on keeping the stock rotors, is the Brembo 4-piston calipers from a 2000 Cobra R. I think they even clear stock wheels.
The best brake upgrade is still better pads and stickier tires, this also happens to be the most cost effective.
This is coming from a guy running stock 13" Brembo plain rotors with the "upgraded" Brembo(Y2KR style) 4 piston calipers.
The functional improvement was mostly less fade and almost no pad taper compared to the 2 piston PBRs.
Actual cold stopping was not a whole lot different.
What wear rating tire are you currently running?
What pads are you using?
And also what are you planning on doing with the car? What car activity?
An awful lot of people upgrade brakes for appearance reasons, which often is not a functionally improved upgrade.