if you change to the later calipers, you do NOT have to change the rotors
been running the 4 pot calipers with non floating discs on my `01 for the last 7 years with no adverse effects whatsoever.
remember, opposed piston calipers were working fine before floating discs came on the market, and funnily enough they still do.
the 2 piston sliding calipers are adequate, but little more. the rear with 4 pistons on a non floating disc are very good, possibly having a brembo master as standard helps there.
using the brakes from the `04 up, or R6 or R1 (the same on all of them ) with braided lines is well worth doing