It seems that with Starling update 0.9.1 their performance are very close. However give the user choices is important.
I've started to work on the simple math-based collision-detection system built, it sounds promising. But, the CE always have to check for Box2D. That means 10 minutes on my macbook pro 2008 to compile an ad-hoc version for iOS (during this time I read an Objective-C book...), which is a pain in the ass.
Added Nape, Chipmunks, ND2D, Genome2D... would be awesome, but the final SWF must be as light as possible and compilation time short.
I think we have to work on the Engine Architecture, and particularly on platformer classes. At the moment I've to rewrite the Hero, for the simple collision system.
We should set up a simple entity/component system so it will be easier to manage all this different settings. However, I've a lack of knowledge & experience on this...