Titan and friends did extensive testing when developing the initial over-clock kernels. The results were a pretty bell-curve that peeked efficiency wise very close to 500Mhz. (It's almost like the designed it to go that fast! ) If you do a proper thread search you may even find the chart laying around still.

I've had mine set to 500 as the base for some time. There were also other cute tricks they tried to "trick" the phone app (changing the kernel speed to 605 vs 600, etc). Sadly, the only real "solution" seemed to be to re-load kernel config after a call hang-up. There was a hack put in to Titan's kernels to do that as I recall, but I don't know if it's still there? I'd assumed Pali just took the bundle from Titan's stuff as a work base, but that may not be a good assumption.