Mac Pro :: Can't The QPI Fetch From Any 3 Of The 4 Modules At Once

I know the newer Intel QPI processors have three main-memory channels, and I've seen much discussion and several complaints on these threads that Apple didn't expand their memory risers to squeeze in six DRAM sockets instead of four, so that the module count on each riser could be a multiple of 3. (And also that they should have done this for the 2009 Nehalems).

Here's my question: what kind of performance would I be losing and why if I got one of these new Westmere machines, discarded Apple's stock memory and replaced it with 4--rather than 3--OWC 4GB sticks? What I specifically don't understand is, can't the QPI fetch from any 3 of the 4 modules at once?