Don't forget that you restore serial, for one client there is only one
restore session, so also 1 tapedrive is needed.
You can save time when you can use collocation, but it takes more tapes; up
to 90 tapes in your case.

So, you can only save time when you need to restore more than 6 clients at
the same time.
An other help could be to use the money for the 14 drives to buy disks and
make a very large diskpool, so you can use caching. All data goes to tape,
but stays also in the diskpool, when you need a restore, this will save A
LOT of time when you have enough network bandwidth.

Good luck,
  Maurice van 't Loo
  The Netherlands