Is there any use for a custom kernel if you're not going to overclock?
For me, one of best feature of custom kernels is possibility to have 640MB data storage (compared to 380MB that phone have with stock kernel).
Overclock is good to have, as an option to have when needed.
There are other features (like multiboot or even to add extra partition on sd card for more app storage) and lot's of tweaks to little spead up phone generaly.