Not sure of your question.
- not doing mailbox backups - good decision, in MY opinion.
However, be sure you check the event logs on the Exchange server and the log file folders after backup runs, to make sure the transaction logs are flushing properly. - don't need to do this all the time, but initially until you are confident all is working properly.
Recovery Storage Group - "disable it" ??
The RSG is a feature of Exchange 2003 that is there for you to use when you need to RESTORE.
It means you can restore all or part of an Exchange SG to a "temporary" area, leaving your production SGs running.
The Exchange recommendation is, that if you use it for a restore, you empty it once that restore is finished.
So, if you follow that rec., it has no affect on backup times.