Well the question is what speed are you actually all downloading? Each seed is a download point, but your not actually downloading the same part of the file, but different parts at the same time. The more seeds, the more parts of the file your downloading at once. Each seed can have its own download speed. but the total download rate is the total bandwidth being use.
So first thing we want to do is rule out some stuff, so connect a computer direct to ISP end point and then crank your Torrent program to full speed. While that is running, open a browser window and do some light browser. Like some google searches. Your not looking so much for lack of speed on the browser, but to make sure you can get on at all. If you get some kind of DNS error, or HTML error, or your ISP devices crashes, stop right there. If not, continue.
Now as for what can be done to improve the router. First shut down "everything" related to QoS. I think you already said you did, just double checking. The reason this is important is your basically flooding your router with tiny fast traffic, so anything that might create a bottle neck might crash it. 2nd, under Advanced>Firewall, make sure both TCP and UDP is set to Endpoint Independent. This will help the router because it won't have to track the traffic going in and out. Leave UPnP on. Then turn off all other computers on the network but one, have that bee hardwired to router, and repeat the test, but at a more reasonable pace. Maybe try for 50Mb down? Then open up a browser and try it out.
PS. Please note the different between MB and Mb. 1MB=8Mb (roughly) So when you report back what your total download speed it from the client, make note of that difference.