I think the most of the performance issues could be solved by offering us methods to load/unload assets & layouts at will and not just automatically by the engine itself.
In my opinion C2's current "dictatorship" with next to no control about the things mentioned above is a big mistake and I really hope, no I count on C3 to finally bring the right changes to this part of the engine.
Personally this is a key selling point for me and I honestly wouldn't consider purchasing C3, if it doesn't deliver any improvements for this.
Just as a side note, I think that C2 is currently the only game engine on the market that doesn't offer features like this.
(Please feel free to correct me if I'm wrong about this.)
I honestly think that give it a couple of years and the whole native vs html5 dilemma will be redundant.
Good luck with the competitors that will offer Native and HTML5 export with a visual editor.
Trust me, HTML5's performance won't be the only thing redundant then (of course this is a reference about Flash).