I mean the thing is revving at 4k at motorway speeds!
Yes the 1.6L 147 I have does just under 4k rpm at 120kmh, the 2.0L in the 155 is at 3500rpm at 120. The 166 is slightly higher revving than the 155 so not all 2.0L boxes have the same drive ratio.
So yes there would be less noise and slightly better fuel consumption, but the problem is that you'll get worse comsumption everywhere else becasue you'll have to load the engine more just to drive around town etc.
Also how strong would a 1.4 gearbox be with a 2.0l lump ragging it through the gears? I know its a bit different but I had a mate creamed an xr3i gearbox that he put into his rs turbo, just could'nt take the torque (or maybe his driving style!).
Luckily with the twinny boxes they are all the same, just the final drive cog that's different, although some have a longer 1st gear too.
Apparently these boxes are good for 200+bhp!
Wanna be careful you don't shatter the 1.4 box shafts as one of the lads I know did taking of rather quickly with his engine converted car.
Yep whenever moving away from the standard there's always extra risk of something going wrong - alot depends on the condition of the box before hand and that the right oil is in it.
Trick is to get a good low miler gearbox.