A question for the general user community on best practices:

     What type of staffing model does your organization use to determine
how many people should be allocated to operate your ADSM/TSM environment?

Background:
It's budgeting season.  Your ADSM/TSM shop has grown in size and you
suddenly (maybe not) realize that you are working too many hours and need
help.  A request is made for additional headcount, but your senior
management team asks how other companies determine appropriate staffing
levels for the work at hand.  How many people does it take to run a "world
class" ADSM/TSM operation?

Our shop:
Two full-time headcount.  Weekdays, but both on-call 24x7.  Responsible for
running/monitoring backups, problem determination/resolution, TSM
server/library hardware/software install/maintenance, client code upgrades,
off-site vaulting, day-to-day operations, training of user community,
budgeting, procurement, testing, etc.  Restores are managed by user
community.

The numbers:
575 UNIX, Novell and NT clients run incremental backups daily, archives run
once per month (adding new servers at the rate of 30 per month)
59 million files examined on the clients for changes daily (increases 50%
per year)
1.8 million files actually backed up daily (increases 50% per year)
1.9 Tb of data actually backed up daily (increases 50% per year)
Backup window from 18:00 until 07:00
403 million files managed on tape (increases 50% per year)
176 Tb of data managed on tape (doubles each year)

ADSM/TSM hardware production instance #1:
rs/6000 m80, 6 way processor, 2 gig memory, 100 Gb database, 5.4 Gb log
10 frame 3494 (with high availability)
12 3590 E1A tape drives

ADSM/TSM hardware production instance #2:
rs/6000 m80, 6 way processor, 2 gig memory, 100 Gb database, 5.4 Gb log
10 frame 3494 (with high availability)
12 3590 E1A tape drives

ADSM/TSM hardware production instance #3:
rs/6000 SP, 2 way processor, 2 gig memory, 20.3 Gb database, 1.0 Gb log
2 frame 3494
6 3590 E1A tape drives

ADSM/TSM hardware production instance #4:
rs/6000 H80 2 way processor, 2 gig memory, 46.3 Gb database, 5.4 Gb log
2 frame 3494
6 3590 E1A tape drives

ADSM/TSM hardware test instance #1:
rs/6000 R50 2 way processor, .5 gig memory, 50.0 Gb database, 5.4 Gb log
1 frame 3494
2 3590 E1A tape drives

ADSM/TSM hardware web proxy server instance #1:
rs/6000 j40, 2 way processor, .5 gig memory

Your thoughts and guidance would be greatly appreciated.