I don't really agree with your 1) for two reasons:
-what you say is true about battery life. But for performances, the kernel doesn't need to "settle down", just be sure to reboot at least once after flashing the kernel is enough to see its performances.
-I've noticed that, for benchmarks but not for the UI, scores tend to gradually decrease with time after a flash. I mean, all my best scores were just after flashing the ROM or the Kernel, and with time (one week?) with no other modifications, scores tend to decrease. Maybe wipping cache is enough to get highest scores again, I haven't verified.
As for benchmarks, I now only use Nenamark2, and give a look at 3D scenes in Quadrant (but not the overall score), the best fps you get there, the best 3D games/apps in openGL will perform. The fps bonus in Nenamark 2, from the Sensation to the One-X, is exactly proportional to the fps boost in Fpse and N64oid for example.