Uh, yes. The graphics benchmarks are extremely biased towards lower resolution screens. Run a graphics benchmark on the 6s and the 6s+ and you'll see what I mean.
No. Even at 1080p test resolution there are many test that iPhone generates better fps or score than a samsung for example. Also off screen test shows you raw gnu power.
http://www.gsmarena.com/iphone_6s_plus_vs_s6_edge_plus-review-1324p7.php