To push more pixels wouldn't the CPU get a small speed bump to match the speed of the iphone 6? Otherwise the 6+ would be heck of a lot slower than the 6. That would def make me not get the 6+ and prob wait for the 6s+.
Perhaps there is a trade off that the A8 in the 6 doesn't run at full pelt as often due to the smaller profile/thermal dissipation compared to the Plus.