They are a good bunch at AMD, it's also handy having Lakeside across the road for a look round the shops
Generally speaking, the MK5 Golf GTI gets to around (realistically) 235-245bhp from a remap, with no other mods.
Perhaps with a cat-back exhaust system, you'd free up a couple more bhp, a full system, a couple more.
But if they are saying 20bhp+ from changing fuel, thats essentially them saying that in total, your remap would take you to ~260bhp
Does seem a little 'over quoted' in my opinion, but that said, turbo cars are so different, im sure some of these GTI's came out the factory pushing more than the quoted bhp...