Well, having never used wu, I'm afraid it was simply a suggestion as an alternative. But, if you check out vsftpd's site, you'll see that full scale websites, certainly in need of robust FTP servers use it.
ProFTPD is very well documented, used by many people, and has quite a bit of scalability, so I'd think maybe you could tweak it to work for you as well, but it might take a bit more effort than say, vsftpd.
A few ideas to look at for ProFTPD:
(for Group allowing)
And the bandwidth limit I think will be the roughest part, at least for me it would be:
Hope that helps