Another suggestion is to download and use
TCPOptimizer (no installation required, just run it). Once it's open, set your connection's download speed at the top and click the "Optimal" toggle (and nothing else; ignore the other settings, for now); I'll bet $5 you see a good number of suggested changes. Go ahead and click "Apply changes", and reboot. I've seen up to 15% better downlload performance and 20-30% uploads at times, just using this little utility.
Note: At any time, you can simply click the "Windows default" toggle to undo the changes.
The documentation is also at that link, but the short of it, is that Windows has very sub-optimal settings set by default. Even in the broadband era, the TCP/IP settings are set up essentially for dial-up connection speeds. This results in, among other things, ridiculously small packet sizes, with correspondingly larger overhead.
Additionally, a secret to TCP/IP is that while multiple connections *seem* like they should decrease data throughput, due to increased overhead, very often this is NOT the case (although there are limits, of course). You'll likely see a performance hit on your PC, before you see a dropoff in the speed bonus from using more connections. With that in mind, I'd recommend a total AND max-per-torrent connection setting of at least 500, if not higher, for your system. Setting the individual limit the same as the max total allows smaller numbers of torrents to still grab all available connections; your client will be able to balance them out if more are running, don't worry (in other words, if you have multiple torrents running, one won't grab all available connections and starve the rest). 15 upload slots is probably not a terrible idea, but make sure you also have set "Allow more slots if below max upload" (or however it is worded in your client), as well.
