If an application starts uploading stuff without my approval then it's a reason to uninstall that software, at least for me personally. That's specific to the seeding part but I guess enabling torrent download by default would be ok if that doesn't also require seeding. Also, seeding might cause issues if people only have limited data per month and automatic seeding would effectively double the data usage of a download (depending on how fast the connection is).
Windows 10, as well as
Blizzard (at least in the
past), use P2P to install their software. Not saying if this is good or bad, just that it works and helps (helped?) them
and others a lot with bandwidth issues.
With all of that said, I'd put the option as a prompt, recommending the user allow P2P, but having the option to disable at launch.
Also, I know that Deluge and others have an option to use a random port, as well encrypt their data stream
, and I would recommend enabling that if it's possible using libTorrent. Some ISPs do indeed punish their subscribers for P2P (better hope you're not using Win 10's new update method or Blizzard's updater then).
As far as upload speed, it's recommended as a rule of thumb to max out seed speed at 10% of max upload speed (and while playing or otherwise using the computer, probably limit it to something like 10 or even 5 or 2 Kb/s, not much, but enough to add up over time).
And for seed ratio (how much you upload), Deluge defaults to 2.00, but I'd also give the option to be a seeder (seed infinitely) for those that want to help out, as well as an option to help out a little but not that much (1.1?), or metered connection (only seed until download is done).
Reason I always bring up P2P is it's the only solution where maxing out your bandwidth literally can't happen, the more people download, the faster they download: your problem becomes it's own solution (vs mirrors ceasing to work when they hit the monthly quota due to massive demand, or similarly, slowing to a crawl if too many are downloading at once).
My 2c, anyways (and I've seen the above happen more than twice with FSO, so it's something that has happened to us before, and during spikes of popularity, the last thing you want is inaccessibility)