If you want to load down from normal power, there are some advantages of going to a faster-burning powder: you'll use less, it will burn more completely, and the recoil impulse will be shorter.
If we take 9mm as an example: Vihtavuori 3N37 is a great choice for full-power 9mm. It's a "slow" powder for 9mm. If you load down too much, it will not burn completely because it won't reach high enough pressure.
N310 is a very "fast" powder in 9mm, but you won't be able to reach the same velocities as with 3N37 because it will reach the maximum pressure much sooner.
Many competition shooters think that in handguns (at least, non-compensated pistols), for two loads at the same power factor (velocity * bullet grains / 1000), the one with the faster powder and heavier bullet will have less recoil impulse. This requires a careful approach because normally you would use a slower powder with the heavier bullets.
I know this doesn't really answer your questions, but hopefully it'll give you some background for working up a good reduced load-- I load "super-wimp" loads of 100PF (115gr at 870fps) and normal powder but reduced recoil impulse (124gr at 1060fps with Titegroup) in 9mm.
-z