You might not get all the 1.5GB back, if some of the mail in this now deleted mailbox was shared with other users. In fact it's possible (not likely, but possible) that you won't get any space back at all if all the mail was shared with other mailboxes, because of single instance storage.
Check your application event log for overnight 1221 events, these will tell you how much white space you have in your stores. I probably wouldn't bother doing an offline defrag for just 1.5GB of space.