I found that my game was becoming to large to run on lower spec systems, affecting the frame rates and causing crashes when exiting the game (pocket PC); this wasn’t a problem with GE, just me expecting too much from the Pocket PC platform. So even though I split the levels so each has its own activation region I have also split the game into three files (10 levels per file) to stop this happening. On the plus side it now means that I can get more variety into each section of ten levels, so it was worth doing
I find that using activation regions, keeping the levels very separate and moving the view around to each one in turn works excellently. Keeping what is being processed to a minimum is always going to be good, so using the activation regions really does make sense, just make sure your actors are not set to receive events even if out of vision (which they default to in my version of GE), as that effectively renders the activation regions useless!
If there are other proven and effective ways to split levels I’d also like to hear it as it may be useful in future projects, however I believe that the activation regions and split files are the best option for my platformer on the PPC