Archive is not about iops. its about streaming bandwidth.

for example I used to look after a quantum iScaler 24 drive robot, each drive was capable of kicking out ~100 megabytes a second. It was more than capable of saturating a 40 gig pipe.

However random IO was shite, it could take up to 20 minutes to get to random file. (Each tape is stored in a caddy of (from memory) 10 tapes, There is contention on the drives, and then spooling to the right place on the tape.)

Email is essentially random IO on a long tale. So, unless your users want a 20 minute delay in accessing last year's emails, I doubt its the right fit.

The same applies to Optical disk packs (although the spool time is much less.)