Brakes

Si brakes are inadequate, although mainly to do with caliper ratio rather than the disc size¹

Front discs

Being an Si, you can fit any 206 or 306 brake package to the front. The most common is 266mm Bosch or Lucas. The 266mm rotor is the cheapest of all, as it's fitted to so many PSA cars.²

You can bolt on brakes from a base model 306 (ie, 1.8l, 8 valve; ATE calipers, vented 247mm discs³), same as S3 205 GTi.⁴ A big improvement and with the pads and discs available you don't really need to go any further than that. However, 306 calipers are getting a bit old, many will be well beyond their service life

Other similar (and in many cases, newer) options include

vented Bosch 247mm rotors and calipers from, for example, a 1.6l 4 door 206⁵ or a 206 with a 1.4l TU3JP (KFW)
calipers and 266mm discs from a Peugeot 306 XSi/HDi (shown here on a 205 GTi) ⁶
vented Bosch 266mm rotors and calipers from later model PSA vehicles: base 207, 206 GTi, Berlingo, Xsara etc⁷⁸

Bigger wheels will be required to fit over 206/306 brakes:

14 inch steel wheels (from a 306/405, not from a 206) fit over 206/306 brakes
There are two types of Pepperpots:
- [Italian] Speedline SL201
- [French] SMR.
French SMR pepperpots don't fit over 266mm brakes. Italian Speedline Pepperpots do fit over 266mm brakes.⁹ It has been suggested this is mainly to do with how deep the dish is on the inside (possibly slightly different castings). The circumference clears (just) but the depth is a problem. ¹⁰
The Speedline variety may be finished better on the inside, with fully radiused holes. The SMR variety may have unfinished, square edges on the holes.¹¹
French SL201 can be made to fit if you:
(a) machine the rim
(b) attack the caliper with a grinder/flap wheel/mill¹²
(c) fit a 3mm spacer until the pads wear down a bit.⁶
for 14 inch pepperpots it is necessary to switch to outside wheel balance weights because the internal ones scrape the caliper.¹³
to fit stock 13 inch wheels over the caliper for vented 247mm discs, you need to run at least a 4mm spacer¹⁴

Rear brakes

The rear brakes do very little in a 205, other than give you a high, firm pedal when adjusted properly.²

Rear drums

Rear wheel cylinders have inbuilt compensators¹⁵
Rear cylinder part numbers are 4402.65 (left) and 4402.66 (right)¹⁴
Si drums are 5mm narrower than gti drums.¹⁶

Rear discs

Pressing 306 stubs into Si trailing arms allows the fitting of 306 rear discs.¹⁷
You don't want rear disc brakes; keep drums, they are more than adequate for the job. Discs add much complexity for no benefit in functionality and a crappier handbrake.¹⁸
Not everyone agrees: Disc brakes are much easier to maintain and adjust. It has improved the feel of the brakes immensely and they don't lock up as easily when you accidentally stab them coming in too hot on a blind corner. Handbrake is arguably worse yes. My mates 306 slowly crashed into a tree on my lawn once from his handbrake failing to hold. If the drums are in good order it's probably not worth replacing them considering the scarcity of conversion parts these days¹⁹