Just to confuse things even more, I tried #Sh3ll from home on a big file and it worked fine (even over wireless).
So that would seem to indicate that the problem is with my network connection at work. Although that still doesn't explain why jSh3ll worked.
However, I didn't get around to trying the others and I only did the one test at home. So it could have just been lucky.
I'm not sure where that leaves me. Maybe we should try #Sh3ll (because it appears to be the easiest to deploy) on a few of our clients and see what happens.