off the top of my head:
1) the 915 is lighter than the g50. less weight is the functional equivalent of moar powar!
2) the g50 clutch is hydraulic, which will complicate the gearbox swap as the 915 clutch is cable operated.
3) the g50 is different dimensionally than the 915, which requires a change in where the rear torsion bars are located.
4) the shift linkage is also different, but i don't think that is nearly as big a problem as the clutch and torsion bar issues.
swapping out a 915 for a g50 is much more complicated than a simple drop-in replacement. the torsion bar change is a pretty big deal; the hydraulic clutch would also be a hassle.
the main advantage of a g50 is that it shifts more like what american drivers typically use. a 915 in good repair, with properly adjusted linkage in good repair, and a driver used to its quirks is a very useful and usable gearbox.
and finally, keeping your 915 in the car will be a lot cheaper
