Thanks for the response, diesieben. (Nice to see you are still at it on these forums :P ) Yes, I do use countEntities, but I use it to limit the spawn of that specific entity class, not ALL mobs. I received reports of modded mobs also spawning less and I cannot even check for those classes so I find it hard to believe that that would be the issue... I'll try to remove it when I get home from work and see, but I doubt it's the problem.
If you find anything else suspicious, please let me know. I'm still stumped.