the biggest difference is the carriers between 256 and 280, the 280 carrier pushes the caliper out enough to fit over the bigger disc.
You need 16v pads to use 256 calipers, but even then may need part worn/ground down pads to get them to fit over the thicker 280mm disc. another solution is to grind the slider pins down a bit, but that leaves you with less 'meat' on the spanner flats so you might struggle to find a spanner thin enough.
If it was me I'd go for proper calipers to save hassle, MK3 GTIs are a good source of cheap 280mm calipers in scrappys now, they're getting common. Just make sure they arnt the 288 ones, as they dont bolt up. Doesnt matter that they're 5 stud either, the calipers & carriers are the same. That goes for most other 5 stud VAG models, SEAT, Passat etc. Just as I say, make sure its a 280mm setup not 288.
You know what to look for though, the 280s are pretty much the same as the 256, girling 54s.