What do you mean exactly by 'backpedal'? As far as I know Carmack has stated that FP in general is achievable for games, he sees the benefit, but the languages and tooling just aren't there yet. There's probably some more research that needs to be done into the ideal mix of imperative and functional features in a language, too, that's something I think is very much not settled. I believe his comments about Haskell specifically deemed it ultimately unsuitable, although not necessarily because of the paradigm. At least that's what I remember him writing and talking about ~8 years ago.
I'd say generally you lose a factor of two in performance when you use immutable persistent collections. Performance isn't everything (otherwise Python wouldn't be so popular) but when people have a system that works the last thing they want to hear is about a way to slow it down.
