Decent stage 1 map from the likes of Revo / rTech etc. should see 240bhp+ and 280lb/ft+ on GTI (K03), if your Edition 30/Pirelli/K04 then 280bhp+ and similar torque.
Decent set of tyres, as above, make a huge difference... Goodyear Eagle F1 AS2's are excellent for daily/fast road use.
Re: MPG, the more power you have the more fuel you will use... that said, driven normally they are no less efficient, perhaps a little more in some respects... but it will drop fast if your enjoying the new found torque these engines produce.