I have done the boy racer thing and that's included such mods as putting a superchip and Performance Actuator in my RS Turbo Fiesta (170BHP) and also stripping cars out to make them faster. As for MPG - the cars I owned at that time had only one thing in mind and that came at a cost of 10-20MPG.
I contemplated a little while ago getting a 106 1.5 and "modding" it in a way that would improve its MPG i.e weight saving, some aero mods and low rolling resistance tyres however it never worked out.
I would be tempted to get some low rolling resistance tyres but that would depend on how much they cost against the gains I would expect. I also understand that superchips improve performance and MPG on TD cars but the last thing I want to do to my tired old Golf is shove another 30BHP and a bag load of torque at the ageing motor.
I don'y often use the radio so maybe I should put the aerial down