have you done a speed test?
and are you sure you have 100mb/s upload? That's quite a good connection if you do.
Most ISPs I know limit upload to 10-20% of the Download speed. The service i subscribe to is 30 mb/s DL but only 5 mb/s up.
http://www.tixati.com/optimize has some information that might help.
I'd make sure the ports tixati is using are whitelisted in any firewalls, especially in the router so it isn't overloaded. And I would think a quality 1 gb/s router would be highly recommended.
Also monitor your system resources. I'd recommend ABPmon and/or TaskInfo for that: iarsn
keep an eye on CPUsage, TCP traffic and disk I/O with those.
I run with about 30 Upload slots with auto-manage enabled seeding well over 1500 torrents total and I can flatline my limit easily, but it is "only" 10 mb/s total up total with a dual WAN connection. If you can actually reach 100 mb/s UP as verified by a speedtest, you're out of my league.
I am not sure how many "plenty" torrents is, but if you have several hundred, I'd try opening 200 upload ports and see what happens.
Keep us posted.
