I've been messing with m0n0wall versions for the last hour.  Here is
what I've come up with so far.

m0n0wall 1.2b5/6/7 all give me the same results with my 802.11 (b
only) netgate card.  I get between 765kbps and 1Mpbps during large
file transfers.  (I used a FreeBSD 600MB+ ISO to test the transfer.) 
The average result was around 850kbps ... it would only occassionaly
drop lower than 850kbps and would only ocassionally go higher than
900kbps.  The connection seemed solid though.  Meaning it wouldn't
drop out and the data rate was constant.  It didn't jump up and down
much at all.  It also did not matter which channel I used.

m0n0wall 1.11 and 1.2b3 give me the same result.  Which isn't too
surprising since they are both FreeBSD 4.x.  But the result with those
was a constant file transfer rate of 5.42Mbps - 5.72Mbps with no drop
outs or sudden changes in data transfer speed.  Channel didn't matter.

So the problem appears to be with m0n0wall versions that are using
FreeBSD 5.x.  Unfortunately there's no 802.11g support in the 4.x
version so I guess I'm just out of luck.

At least for now, the 5.x FreeBSD versions aren't an option because
the wireless connection quality is just too horrible.

I'm not sure what else I can do.  I don't know if I can return the
802.11g netgate card or not.  I certainly don't want to.  I want
802.11g speed.  But I don't know when or if these apparent problems
will be worked out.  If it's m0n0wall, it might be fixed in the next
beta.  But if it's a kernel problem, there isn't much Manuel can do
about it.