Hi Mdrew,
As JohnK pointed out (thanks, John), there aren't any downsides to using an identical speaker (to your main left and right front speakers) as a center. Keep in mind that even when you do use an identical speaker, it won't necessarily sound exactly the same, at least with pink noise, as the left and right speakers simply because it's in a different location.
But it would certainly have identical power handling, dispersion, frequency response and so on as the main speakers. If you have the budget and space for one, sure, go ahead.
On the other hand, it isn't a big compromise using a well-designed center channel speaker whose timbral signature smoothly matches the main speakers. You aren't going to "lose" bass frequencies when you configure the system with a subwoofer, as the ".1" channel routes all the deep bass to the sub or the main left and right speakers when you set the center up as "Small".
The other option of using dual VP150 center channel speakers, one above the screen and one below, running in parallel, gives you tons of power handling, excellent coverage for all listeners, plus the advantage of the two centers imaging the dialog and vocals at the center of the screen.
Regards,
