Torrenting works fine with Mullvad in my personal experience, and will pretty much up to my current ISP speed limits (which is 200Mbps download).
Can't really guarantee you that it will be as good if you're hosting your own seedbox over their VPN (then again if you're doing that you should probably pay for a proper seedbox hosted elsewhere) but if you've downloade something and the just leave it seeding, it seems fine.