Radios are flashed to your phone separately from the ROM, kernel, and recovery. Flashing a custom ROM won't update your radios, but flashing an OTA update package would change them if the OTA included a radio update.
Here is
zero's thread for the Galaxy Nexus radios, and
here's the one on RootzWiki. If you're already on the most up-to-date radios, there is very little you can do to improve your signal strength other than by removing interference/relocating yourself.
Edit: something to keep in mind is that battery drain corresponds more to raw signal strength than it does to whether or not you're actively using the data connection. The biggest hit will be a poor LTE signal -- I would suggest simply switching LTE off if it is a significant problem.