We have the opposite situation - we have fast robotics and use collocation.
With collocation on fast tape, it doesn't matter whether you are doing 2
weeks or 2 years of data, a restore takes the same amount of time.

Doing periodic fulls doesn't "refresh" anything, from TSM's point of view -
the original backups are still in the TSM DB and still available, even if
they are 5 years old.  If you do periodic fulls, you have to retransmit
everything over the network again, and you have to adjust your policies to
make sure you allow those redundant versions to be kept; you increase the
size of your DB and the amount of reclaims you have to do.

Doing periodic "fulls" would do nothing whatever for us, except bog down the
network.

I suggest you try doing a large restore to test your own capabilities.  If
you can't restore in a timely fashion, FIRST figure out what your bottleneck
is before you decide to "fix" it by doing full backups.

Then if you find out you still can't do restores in a timely fashion, at
least check out the use of BACKUPSETS.  They give you all the client's
active data on one tape, without retransmitting all the data, and without
creating an extra zillion entries in your DB.