Most time I use GL renderer along with standard openfl DOM renderer (which renders images / divs / canvases). In this case externalizing render method is possible, but will add additional complexity.
Indeed, if you targeting flash with Stage3D, you can do some optimizations, for example instead of having multiple Tilesheet’s for each texture use only one Tilesheet, externalize render method, etc…
I want to keep library as simple as possible, and it should work well on all targets, not only flash.
Probably we can make ParticleSystem more renderer-independent or even engine-independent (by removing BitmapData from it). In this case you can use ParticleSystem (which will take care of all math) with you own super optimized renderer, and not only on openfl, but on lime, show or luxeengine.