The volume cap on European iPods is implemented in software by the OF. It doesn't apply to Rockbox at all.
The biggest problem with Rockbox on iPod classics currently is the relatively rudimentary boot and install procedure. The Rockbox iPod loader has not been ported to the classic yet, and so you have to install
emCORE manually. emCORE cannot boot the original firmware, so what you have after installing Rockbox is effectively not an iPod. It can no longer sync with iTunes, for example.
If you need to be able to sync with iTunes or use the Apple firmware, you should not use Rockbox on an iPod classic.