Dummynet low bandwidth simulation

I'm trying to see how a web server performes when "attacked" by many low 
bandwith connections, and my idea is to use dummynet to simulate such 
conditions from a single client computer on the same (100Mbps) LAN. I've 
created a pipe:

ipfw pipe 1 config bw 56Kbit/s delay 250 queue 20 buckets 1024 mask all

My rules are:

ipfw pipe 1 tcp from me to webserver dst-port 80
ipfw pipe 1 tcp from webserver 80 to me

When I start the benchmark program (siege) with relatively low 
concurrency (50 clients), everything works as expected, but after 
increasing to 100+ simultaneous requests, I start getting huge amounts 
of "read error: Connection reset by peer" and "broken pipe" messages. 
During that time the load on the web server is low (plenty of idle time 
and memory).

Without traffic shaping, the server handles 250-300 concurrent requests 
without problems in the same setup.

I've tried changing the queue and bucket size (and adding 'noerror'), 
with no results. Eliminating latency (delay 0) "fixes" the problem, but 
it's not meaningfull that way.

Am I missing something?

-- 
C isn't that hard: void (*(*f[])())() defines f as an array of
unspecified size, of pointers to functions that return pointers to
functions that return void.