Are you rooted?
If your rooted then yes you can keep the rom with a new kernel.
The fastest one is xtreme kernel. Its overclocked to 1.6 ghz and has the frame rate unlocked.
Battery its really all comparible. I've heard that the Phoenix is the best for battery.
If speed AND battery are what you're looking for, many people have reported (and I agree) that Baked Snack has excellent battery life. It is also super quick. There are 4 variations of the current Baked Snack kernel out, each one having an increasingly lower voltage for better battery savings. However the voltage is so low that all the kernels do not work for all phones.
I'll list them here, highest voltage to lowest voltage (least to best battery life): babystep, step 1, step 2, ultimate. The developer suggests start with babystep, and if your phone runs fine without freezing you can try going to step 1. If step 1 is fine, go to step 2, etc. You'll know when the kernel doesn't work with your phone because it will freeze up, requiring a battery pull. You can download them here.
Also worth checking out: Xtreme kernel, Phoenix kernel, Mixup kernel.
1) Install ClockworkMod Recovery
2) Make a backup of your phone using Clockwork in case anything goes wrong
3) From Clockwork, select the kernel zip file on your SD card and flash it
yeah i got it now but still dont know what kernel to go with. let me know people i want better battery life and speed. ragnarokx ur using step 2? hows that working for you? battery life?
it didnt work!! phone wont go pass samsung start up screen!!! tired step2 and than babystep. can someone tell me what i did wrong??? on stock rom and never used kernel.
just to make sure i know i dont have a rom on my phone using the stock sprint one it comes with so can i or cant i use a custom kernel??? and if so which one works 100% of the time?? Build:sph-d700 eclair.di18