put it on a dyno and check the basics. so many variables and so many things changed.
everyone is quick to blame the workshop so before you go doing any of that, get it all checked and check the basics on the dyno. check the tune, check the timing, check the fuel, check the fuel pump (walbro's can be ass sometimes) check the AFR's
excessive fuel usuage is usually
poor tune
poor closed loop
constant loading the car up / bring it on boost repeatedly
a good check is to give the car to your girlfriend or mum and let her drive it for a few days
i bet the economy doulbes or triples when they drive it (ie they baby it and dont bring it on boost constantly)
i know you said you dont do it, but its a simple mechnical check, if if they get same ass economy its not your driving