Hi as chief mentioned above, the C2 has 2 piston calipers on the rear, the C4 as S10lemon has on his post above has 4 pot Brembos on the rear which look identical to the front calipers on the C2 but with smaller pistons.
In my opinion that's the best first step upgrade.
The 44mm pistons on the C2 rear calipers actually have a greater brake force on the pads than the 28/32 pistons on the C4 rear calipers for a given hydraulic pressure, but that's only half the story.
I found that on track the tiny pads combined with the standard C2 calipers cased the pads to overheat causing fade and warped disks.
The rear pads on the C4 rear calipers are much larger (same pads as C2 fronts) so there's more surface area.
If you change to C4 rear calipers you should also change the brake bias proportion valve from the C2's standard 45 to the C4's 55 bar valve.
I think I'm correct in saying that the 944 S2 has the same rear calipers as the C4 964 as has the 928
So it gives you more option when looking for a second hand set.
I got a set of 928 rears from the US from a west coast breaker yard and they came delivered in great condition (no salty roads there) and they didn't cost much.
The next upgrade up from that is to go 964RS or 993 brakes which will require bigger disks and calipers all round and the costs will increase considerably.
With the above C4 upgrade despite "spirited" driving I've not experienced any brake fade on a public road, the brakes are very capable.
Hope this helps
Logan