I use fastsend and only time anyone has had a problem getting any file from me has been modem users... and even then, it's not often. And I haven't had issues myself except back when using a modem. I think I'd suggest the rollback script for modem users to be something like 16k and leave it at 8k for broadband users.
I really don't remember a whole lot about modems, but I think the way modems handle data, a disconnect on their side can cause a loss of more than one or two packets (causing the corrupted file).
