The reason there's a difference with Airplane/Data mode ON/OFF. Is because that state affects "sync" of most (if not all) of those background tasks.

And the kernel "forces" them to pause, allowing Android to go into Deep Sleep and save you battery. I can confirm this is the case with my one too.

However Airplane mode isn't really a solution for most people so you sort of need to put up with it. Like I said slowly the memory leaks are being discovered and patched, so a better performing kernel shouldn't be too long away (hoping).

Unfortunately, I cannot code for kernels. Its really really difficult to do so (correctly, unlike half of android kernel devs). I say the kernel is the most important part of your custom firmware, followed by your rom (higher order operations), then radio (patched from oem), then drivers (rarely ever improved).

I'm beginning to slowly understand Linus, and appreciate what they've (not him individually, but part of his group) achieved. Version 3.5 can't get here any sooner, or ARMopenSUSE v12.2