For the heck of it, I queried KEF America from their website, asking if a slightly larger iQ center was ever considered. I pretty much got the response I expected.
That is a good idea, however we have found that people are interested in smaller slimmer center channels that fit inside holes and shelves in and entertainment center. For that reason we stuff our less expensive center channels with 5.25" drivers to keep them slim.
Have a good day
Parts & Customer Service
Not that a center with all 6.5's would be that much larger..
Personally, I think there would be a market for both.