Actually, compresssion isn't the problem.. the problem is with getting data from your server to the tape drive. You plan to do 10Gb between the devices? If not, you are never going to get more than 133MB/sec since that's the maximum througput of Gigabit. FreeNAS doesn't support any tape drives at all...
To sum it up, you're looking at something that involves 2 machines(FreeNAS + a desktop), with then connected via a LAN of some kind doing the backing up to tape, that you'll have to change out every 4+ hours or so, then do the next tape until it's done.
The reality of it, you could probably build a whole second box to do ZFS replication to, have it handle everything without joking around with Tapes, have the convenience of instant access to the backups whenever needed, and is probably far more reliable since you are using ZFS. ;)
Big picture, we've seen people propose the tape idea before. And for big pools, using tapes is not economical, not logistically sound, not fast, or all of the above.