If your Exchange Stores backups use GRT and reside on disk, then you can restore directly from disk. If the data has been DUPLICATED to tape (which even with HF14 can take an obscene amount of time) and removed from disk, you need to either Duplicate it back, or have enough temp space to restore the entire Stores backup, which then extracts the desired data.
Or you could indeed use the RSG
Or you could choose the Legacy method of backing up Mailboxes and restore directly from tape, but you then run into the whole can of worms regarding the extremely slow throughput problems.