Ken's got a good point, but it's fun going overkill - especially with someone else's money! Now, how would you go about with a quad-center channel arrangement... ?
Seriously though, 3 M80's or other good vertical speaker equivalent across the front would be the ideal. I think adding to that might start to make the center channel sound different than the L&R channels which defeats the purpose of this exercise. The double center idea makes most sense to me when you're asking smaller center channel speakers to 'keep up' with larger L&R speakers. If they're all already equal, I am not sure what you'd gain by adding another speaker.
