You want to say that if all miners establish the highest possible exchange rate among themselves, this makes the system more centralized. Judging by the number of orphan blocks, the miners have achieved this goal. That is, at the moment, the miners are as centralized as possible in terms of the exchange between them. It is simply impossible to make it more centralized already. 
Hmm. I recall certain pools don't have direct connections to each other and were resorting to running zombies on each other in the past. I find the current stale rates fairly reasonable and mining pools don't necessarily have to resort to those tactics to improve their propagation. If we were to reach the state whereby blocks takes 50 seconds to propagate, then is it possible for the major mining pools to strategically exclude others from their exclusive "subnet"?