EDIT: I'm not quite sure what you mean by 150 extra objects? When I tested your cap 1000 extra sprites where spawned at max. Or do I get it totally wrong?
That's strange, guess your using unlimited framerate?
I'm using v-synced at 60 fps, and it never drops below 70 fps, and 150(ish) objects.
Its also using 4 megs of vram for the one additive fx, another reason I'm not sure if it's even a good idea.
I mean the spawning effect looks ok, other than the occasional skip. And thats really the main reason I thought an fx might be better.