I had my GT done the other week. Mine has 72000 on the clock and when he got it above 280lb/ft of torque the clutch slipped! He also found I had a problem with the vanes on my turbo sticking, not enough to give a fault but a problem none the less. So at very least I'd run some good quality turbo cleaner (like Forte) through your system or clean the turbo with some Mr Muscle before you go. Apparently you can do the Mr Muscle option with the EGR valve removed without having to remove the turbo, although I haven't tried it!
I asked for my map to be based around MPG as much as anything else. I did a couple of runs on known routes before the remap, and again after. On motorways at aprox 65/70mph(with cruise control on) I was getting 51mpg(average), after I get 56mpg, which is aprox 10% increase. But on B roads at around 50mph I can get a little over 15% increase in mpg. You have to drive it with economy in mind though.
Also had EGR valve swithed off while I was at it.