I just bought Titanfall yesterday. I run this on a watercooled PC, dual GPU setup. I wouldn't have bought such a PC setup just to run at 60% of system performance. I think Mozilla is marketing this wrong. Triple-AAA titles go for squeezing every last ounce of system performance. I don't think there is anything to be gained to putting a game like Titanfall, which has 5 gigabytes of assets (some UE titles are much bigg…
But the important thing in working to port the highest-end games is to push the limits of the web platform, so it can be better optimized. Before UE3 was ported, people were - rightfully! - skeptical any such engine would work well on the web at all, and that port helped get to that point (and now to UE4).
Also, note that the 60% is of CPU performance. You also get close to 100% of GPU performance using WebGL. So overall it will be even closer to native speed (depending on how the specific game balances between CPU and GPU).