are you more concerned about the closer transmission ratios or the final drive ratio? it really isnt worth your trouble to change the actual transmission ratios unless you plan on competitively tracking the car with a moderately high budget, the final drive, differential, ratio is what most people go after. without the influences of air resistance and power/weight ratios a shorter diff ratio will allow you to accelerate quicker but sacrifice top speed where as a taller diff (smaller number) will gain top speed but slow down acceleration. and like jason said, a torquier more powerful motor is better suited to a taller diff, for example (i'm not sure if these numbers are 100% accurate but theyre close) a 330ci runs a 2.93:1 final drive ratio and a 328ci runs a 3.46:1 ratio, which makes sense given the smaller engine with the same transmission needs shorter gearing to compensate for less power.
changing the diff will just shift the engine to road speed relation into a shorter or taller(longer) distance. so ideally a good upgrade would be to go with a shorter diff, but add another gear (pretty much just for fuel economy reasons as you may not even be able to top out 5th gear, much less 6th)