the overall big picture here is you are worried about heat / power usage etc. the issue is not about installing torrent junk on pfsense. so get to the root of the problem
a 1200mhz duron wasn't designed to be a 24/7 routing chip, its a desktop processor u could cook your eggs on with big noisy fans etc
yank the power sucking, heat generating firewall pc. put pfsense on a silent passive cooled embedded device with compact flash.
then get another one and install linux and run it just for file serving/torrents
theres even 1u rackmount cases and such that will accommodate two separate mini itx motherboards, then you can have your "one pc" powered on 24/7 but have two independent pc's