[CS3] copyPixels + Sprites/Containers questions

I've finished converting the particle engine in my game to a copyPixels + spritesheets + 1 screen sized bitmap system, while the rest of my game is done with sprites and a big container for the world. This is my first time doing copyPixels or any direct bitmap data manipulation so I don't really know any standards or anything beyond what I've found on various blogs. Is it ok/normal to do it this way? I end up using localToGlobal for every particle to determine where on the bitmap to draw, based on some collision or whatever that took place inside the container. Isn't localToGlobal slow? Should I be using a system of scrolling containers that are the same size as the screen? I didn't really want to use copyPixels for the rest of the game because it performs ok, isn't really graphically intense, and the particle system doesn't have any collision or even rotation so copyPixels is really easy. Any advice would be much appreciated :)

Edit: The game, incase it helps, is here
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That is only fast way to use large number of particles. With small enough stage size FP10 can handle 10000 moving objects using this method.

Rather than use local to global, why not a use a simple offset based on where in the level you are?