I've pulled my centre speaker forward on my rack. It still sits about 40-50mm further back than the L and R speakers, but is now only about 25mm from the front of the rack where it was about 75mm.
It's really helped with movie and TV sound, but not in the way I expected. I suppose I was expecting the voices to be clearer, or louder or something. But the actual effect has been to make the sound all around the room much more of a whole - more coherent and less like its coming from individual speakers.