Nah. The A8 will (hopefully) be clocked higher, because it NEEDS to be to drive all those extra pixels. Think iPad 2 vs iPad 3, where the iPad 3 'looked' better due to retina, but was thicker, ran very hot and performance wise was inferior to the iPad 2 in some cases.
No doubt the 6+ will look more gorgeous, with that larger, more retina screen. But it's going to run warmer than the 6, and I'd expect performance to be no better than the 6, perhaps inferior, a la iPad 2 vs iPad 3.