We already had have rudimentary support for source maps - in fact it seems people have already used it to successfully debug their programs. It's not good enough yet for step-by-step debugging - we're getting there.
RE: performance. I don't know who you talked to, but they are misinformed we spend a ridiculous amount of time on performance. It's certainly possible to do an HTML 5 canvas game in ClojureScript - http://swannodette.github.io/2013/06/10/porting-notchs-minec.... It is true that if you want to do a computationally intensive game you need to know something about JavaScript performance - but that's to be expected.
This is a fantastic tutorial - http://github.com/magomimmo/modern-cljs
RE: performance. I don't know who you talked to, but they are misinformed we spend a ridiculous amount of time on performance. It's certainly possible to do an HTML 5 canvas game in ClojureScript - http://swannodette.github.io/2013/06/10/porting-notchs-minec.... It is true that if you want to do a computationally intensive game you need to know something about JavaScript performance - but that's to be expected.