Slight correction:
Stock diff rings are made from basic 'Cast Steel', and the upgrade diff rings are made from stronger 'Machined Steel'.
You would think fitting the little heavier machined rings would slow up your take off speed, but because the teeth are truer you actually get more take off speed.
The Kawahara diff rings are lighter and finer pitched and run even freer than the machined gears, so you lose nothing fitting these overgeared rings, and you never run out of revs as you use the smaller pinions.
I have just started using the Kfactory pinion gears and they appear to mesh better than the stock gears, hopefully they will last a little longer.