I can't answer the MSDP space question, except that it has seemed to me that the dedup rates for Exchange data are very high.
To GRT or not to GRT:
It makes only a little difference in data storage. The GRT phase replays the Exchange transaction logs against the backup image of the database, then stores the change in the .edb file as a "delta."
The GRT phase of the backup takes some amount of time. I don't have numbers to give you.
The backup stores the mailbox names in the NetBackup catalog files file (a.k.a, the .f file). That's one file entry of several hundred bytes per mailbox.
Duplication of the image extends the catalog files file to include an entry for every mail folder, message, contact, etc. You can turn this off when you run a duplication, whether by SLP, through the GUI, or on the bpduplicate command. The reason the default is to catalog the mail content is so that if you have duplicated to tape and expired the disk copy of the image, you can browse to find the image you want without duplicating lots of tapes back to disk.
GRT is essential for saving time when you need to restore a few folders or messages, such as someone's Inbox. Before GRT, administrators told me they spent most of their days restoring databases to RSG (precursor to RDB) to harvest users' mail.
GRT is not the right tool for restoring many MB, such as an entire mailbox. For that, restore to RDB and get the mail from there using Exchange tools.
You can create content that GRT can't restore, due to Microsoft limitations in Exchange Web Services (EWS). Microsoft began deprecating MAPI outside of Outlook 10 years ago. EWS was supposed to replace it, but they didn't implement everything MAPI can do. Two things I know you can't do and have GRT restore the item are, 1) embed an OLE object in Rich Text Format (RTF) - your users should be using plain text or html format - and 2) embed non-printable characters in text. That's very hard to do, but I've seen customer Tasks whose titles include vertical tabs. I have not been able to reproduce it. Just don't do it.
