The 40 user limit does not apply to all geographies.
That's true, we got an 515 for 50 users (internal) and unlimited (external)
to replace an old 270 (150 CPW). We've also studied other users
combinations, if I'm not wrong the limit is 100 internal users, you can also
buy new licences on the future (groups of 5).
The speed change on our applications was amazing, really a couple of users
complained that some jobs are just running too fast (ie: about 4-5 minutes
are now close to a second).
I don't know about your app, users and disk usage, but right now, to us,
that 515 is many times faster than needed.
Some basic servers specs for comparision:
270: V5R3, 1GB Mem, 4 x 9 GB disks (10 KRPM)
515: V5R4, 4GB Mem, 4 x 70 GB disks (15 KRPM) with RAID
Regards from Brazil,