In terms of one single mod, wouldn't a live remap (to iron out any fuelling blips etc) on existing set up be sufficient to improve economy? So looking at around £250 ish. Will usually improve drivability too. Flea's your man assuming he's still doing it.
Sounds like your mpg is pretty good already, but should be possible to achieve a bit more.
It certainly helped with mine.
I took my stock 20vt6 to Mr Flea a couple of years ago after she had been saved by Countrycruising. He recommended flea-and told me that a good live map makes the coupe come alive. I've always been wary of "remapping" over the years. Read all the horror stories...but my mpg in the coupe with the taps open was quite dismal. So I went to Mr Flea.
He asked me how I wanted the car-and I suggested a "Fast GT Car. Good MPG. With good torque"
Watching a live re-map with someone who really loves the coupe, on a rolling road, with the engine being worked is really something to watch, see and feel!
Around 1.5 hours later I had a transformed coupe. A small up tick in performance at 268ish bhp from a stock motor. No fuel pump/mods.
But in the road...wow. More performance then you could ever deploy. Torque everywhere-so you can drive the car 1st/3rd, 4th/6th etc off boost and not needing to tickle the turbo. Or deploy the turbo and see the countryside start to fly by.
MPG readily improved. With full tank from middle UK -my last eu adventure found me pulling in for a considered fuel stop with a 1/4 of a tank of gas left at the border of Belgium in to Germany. So 3/4 to get through half of UK, across france, and down through the whole of Belgium.
The return-including laps of the 'ring - found the car safely back home in blighty-with jut under 1/4 of a tank.
Sounds too good to be true but I got an uptick in performance that you can feel clearly- AND lower fuel costs.
Would recommend a live tuning session with Mr Flea without any hesitation. Your missing something if you have not done this imho.