I set The Evil Within and Shadows of Mordor to 30 fps. I would prefer to have the performance boost.
All you're doing is maintaining performance consistency. Because your framerate never goes over 30 fps (and assuming never below) you get a consistent visual experience throughout the game. Personally I love it when I get bursts of 60 fps in areas where my PC can handle it, it lets me know what I'm missing. I still remember the first time I saw 60fps rendering, playing GLquake at a buddy's house. My poor VooDoo system could only do so much, and I was totally used to it. He got a brand new 400mhz PII with VooDoo2 card and kablamo, I was thrown off-guard. He even laughed at me wondering what the hell I was marveling at, since this was standard for him. I couldnt explain it, I just kept saying "it's so smoooooth!!" That, and I fragged like I had never fragged before.