Well technically we have seen that if done right and from the ground up, RT can in fact perform better e.g. metro ee.
Avatar is due this year and RT only so it will show how perf. is. I'm expecting good things given it is by massive on the snowdrop engine.
Not that we'll see any fundamental leaps in performance - but in a lot of cases now with games using some ray tracing they've got all the performance hit of a conventional renderer then a large part of the overhead of a ray tracing/path tracing implementation just to use a few token RT effects. With Metro where they use much less conventional features and lean more on RT the performance impact is a bit different.