Just a thought. As the c5s run the handbrake on the front calipers would it be possible to use these to upgrade the rears on the 306? Obviously a bracket would need fabricating to mount the caliper and some modification may be needed for the handbrake.
Should allow both 266 and 283mm discs to be used depending on what calipers are used as both were fitted to the c5 from the factory using the same bracket.

Although I am thinking piston size could be too large?

think there's a kit already for a larger rear brake upgrade, seen it on someone's car on here, possibly puglover's years ago. think they used a bracket and s16 front calipers

cituning are the ones who did the kit i was thinking about

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306 rallye reproduction decals (full sets/individual decals) | 306 b-pillar textured vinyl weatherstrips
306 slam panel esso stickers | 306 yellow / orange / pension fund red side door badges
gti6 inlet manifold badges | 306 rear boot badges (p2/p3)
winner of Extraction of toys from prams with outstanding vigour award 2009 [source: gti6 owners club]