Hey,
i have a tuning box on mine, and the power/torque improvement is marked, and fuel economy is significantly better, +5mpg or so (but thats only if you drive the same as you did before the box, if you exploit the extra power then you wont do very well haha)
i can manage about 55 mpg doing about 85 mph with mine.
Having said all that i just picked my car up this morning having had a new clutch fitted....i put the box on about 15k miles ago. i only bought the car at 50k miles on the clock though so i dont know if it was me or the previous owner. Im not sure it was me though because the clutch never felt like it does now....
anyway, i'm a fan of it overall. and it passes its emissions tests at MOT with ridiculously low numbers so its not overfueling and it wont fail that sort of thing