I've never flashed a custom kernel or ROM, but wouldn't you just restore a Nandroid backup. Or would this not revert the Kernel?
I want to know this for sure! Before I flash custom stuff. Also, can I try a custom Kernel before trying a custom ROM?
I run Bugless Beast 0.4 custom ROM currently and saw about a 20% increase in battery life going that route, and my Quadrant score jumped from 850 to almost 1100. I JUST decided today to try going to a different kernel and am trying a LV Chevy 1 ghz. I just checked my Quadrant at 1381...which is my highest so far, and the phone definitely feels very smooth.
I have had it unplugged for 3 hours and have run two Quadrant tests and played a little music and it is down to 90% so it seems quite promising.
In response to the question about the "stock" kernel, I don't really know why anyone would want or need to revert back to a truly stock kernel. As others have said, download a number of different kernels on to your SD card in case the kernel you want doesn't work, then just pick any 800mhz kernel that works on your phone to "act" stock.
Yes, a Nandroid restore would put you back to whatever kernel and whatever ROM you saved.
Also, I think ROM Manager has "Stock images" that you can download that should be a stock ROM and kernel.
Yes, I do believe a custom kernel can be tried before a custom ROM.
The only real precaution is to have several different kernels of different voltages and speeds available in case your chosen kernel gets you stuck in a boot loop.
Have fun.