I made a quick edit to allow multiple focused BGSes at once. (It is NOT ready for release, just a test to see how it would work.) I have sixteen focused BGSes running at once (all triggered by a single event, but in different locations), which do still work as expected with regards to volume and pan, and not a single trace of FPS slowdown.
I could add more, but really, if you have a need to run sixteen BGSes simultaneously, you might want to step back and ask yourself why.
Now, granted, I was running my gaming PC. Might want to be more cautious with mobile-targeted games.
EDIT: I raised that number to about 200 without a problem. I raised it to over 1000, and... well, okay, now it had about half the framerate as usual (I turned the volume on the events and my speakers WAY down before trying any of this!!) Again, you should never ever ever need that many.
I can't imagine that SEs are likely to cause more memory slowdown on their own. The events themselves are more likely to cause slowdown.