The cost of a replacement 'box (plus be daft not to do clutch and DMF at same time) would wipe out any MPG savings even if it's possible, It's not a 306 with a £50 'box unfortunately. Got a feeling the 'box on mine is dying though, getting a bad whine from the 'box at high rpm and rundown, same as I got on the TU 306 'box with knackered diff bearings. 
I know I'm a bit hard on the drivetrain, power through corners, engine brake a lot and take bumps harder than I should but the driveshaft should be taking that abuse rather than the diff surely! Then again, I do hear potentially bad noises all the time, so it may be nothing at all lol. Doesn't help that the Focus has less sound deadening than a 306 so you hear literally everything!
I know I'm a bit hard on the drivetrain, power through corners, engine brake a lot and take bumps harder than I should but the driveshaft should be taking that abuse rather than the diff surely! Then again, I do hear potentially bad noises all the time, so it may be nothing at all lol. Doesn't help that the Focus has less sound deadening than a 306 so you hear literally everything!