Welp, this is an "I have to" situation... because I ride a Bighit.. so it's got a 24" rear, and a 26" front.
I honestly don't feel a slow down over any of my friends BIG Dual-Suspension bikes... on steeps, decents, or in urban rides.
If you put a 3" Gazzi on the back of a bike that's running a 24" rear.. it'll make the diameter the same as a 26" simply because the tire's so beefy.
I have an Arrow Racing 3.0" on the rear of my bike right now, and there's probably only a 1/4" difference in Tire Diameter from the 26" to the 24" ..
For wheelie drops, hucks, big jumps, and DJ's... the 24" rear feels AWESOME.