It's a while since I've driven a 3.0i with five-speed manual but I remember the gearing being pretty sweet. The higher final-drive ratios may not make for better fuel economy and you will find that you'll be changing gear more often in hilly country or give-and-take driving. The factory goes to great lengths to optimise the gearing for each engine. If you can't afford the update 3.0i with the six-speed manual I'd just stick with the five-speeder as is. I think you could go to a fair bit of effort in changing the diffs and end up with something that's inferior. That my two bob's worth anyway...