"GC-ed languages are going to have to include the GC which I doubt can compete with JavaScript."
There's no particular reason why not. It's all just bits and bytes in the end, and asm.js gives a pretty low-level view of the world. And if you're starting from a baseline of a language that can easily be 5-10x faster than browser-based JS you can afford a bit extra on the GC side.
Javascript isn't magic. It's just a language. It isn't even a particularly special one, once you ignore its browser support, and it certainly isn't one focused on performance (I stopped buying the "languages don't have performance characteristics" line a while ago). It gets to run the same assembly instructions everybody else does. It isn't as fast as a lot of people here suppose, and it isn't that hard to beat out its performance even now.
Why are you asking as if it's some sort of theoretical question when asm.js is in hand, right now, and it performs wildly better than raw Javascript? Of course we'd rather compile to something that's faster than Javascript than compile to Javascript.
(Sorry, I can't condone the word "transpile". Usage of it just reveals someone who doesn't understand compilation technology and thinks there's somehow something "special" about compiling to one intermediate language ("javascript") vs. another ("assembler").)
I can't believe how many people seem to believe that Javascript is a C-speed level language, and downmod anyone who observes it's not. Well, it's still not. It's easy to see that it's not. It's not even close. If it were asm.js wouldn't exist. (I mean, if you're having trouble with my claim here, stop and think about that for a moment... if Javascript is so fast, why does asm.js even exist?)
There's no particular reason why not. It's all just bits and bytes in the end, and asm.js gives a pretty low-level view of the world. And if you're starting from a baseline of a language that can easily be 5-10x faster than browser-based JS you can afford a bit extra on the GC side.
Javascript isn't magic. It's just a language. It isn't even a particularly special one, once you ignore its browser support, and it certainly isn't one focused on performance (I stopped buying the "languages don't have performance characteristics" line a while ago). It gets to run the same assembly instructions everybody else does. It isn't as fast as a lot of people here suppose, and it isn't that hard to beat out its performance even now.