Really haven't noticed a huge difference outdoors, I placed my 2 phones side by side (6+ and 8+ with same brightness settings, the 8+ looks SLIGHTLY dimmer on my end. Could be the new true tone color?)
Both running on IOS 11.1, As for speed and battery...that part is night and day. Opening apps, Safari, Email, App Store at the same time on 6+ and 8+...wow. Everything on the 8+ opens IMMEDIATELY while the 6+ takes a good 2-3 seconds for anything to load and show up. The battery life...again 8+ battery life is incredible so far, while my old 6+ - with a NEW OEM Apple battery I bought in late August (previous one was starting to bulge and I had to replace it ASAP), has suffered greatly after I installed IOS 11.
Right now the 6+ is simply a spare WiFi unit. Using it as a backup HD video cam, camera. device for my drone, a remote.