IMP performance under load

[Stephen Fischer]

I'm evaluating IMP, along with a couple other IMAP web mail clients, and have
just talked to someone who was experiencing a very high load on the machine
running the web server.  He said that they were seeing a sustained load of
about 100 running IMP 2.2.4 on a dual-processor SGI 3200.

I was wondering a couple things.  First, has anyone else had experience with
SGI machines and has suggestions?  Second, has anyone seen this sort of load
in a solaris environment, which is what we're going to be running here?  And
third, I would like to see what sort of hardware you're using if you have a
user environment that is at all close to ours:

    ~40 000 users 
    expected 150-200 concurrent users, although that guess could be wildly
        off since we only have POP usage stats at the moment.

Any information would be of great assistance, particularly since it's hard to
stress test a system like this without actually putting it in a production
environment, and by that point it's a bit late to turn back.

Thanks,
Stephen