Evo magazine tested different fuels in a GTI a few months back, and I've listed their results below (brand, max power, max torque):
Shell 95 205bhp 234lb ft
BP 95 204bhp 242lb ft
Shell Optimax 209bhp 242lb ft
Tesco 99 212bhp 242lb ft
BP Ultimate 212bhp 252lb ft
Sunoco Race Fuel (352.5p/litre!) 218bhp 248lb ft