What's your computer specs (mainly CPU)? Minitheatre releases seem like low complexity encodes to me, plus yes, higher filesize (and thus bitrate) does mean more stuff for the CPU to handle. I'm actually trying to get one of their files to check but holy crap their download options are terrible, barely 100 KB/s.
Considering your issues with other playback setups and madVR, I'm willing to bet your computer is the issue. A fresh install of CCCP and enabling LAV Filters for H264 playback should probably be the most efficient setup, maybe replace VSFilter with xy-vsfilter to reduce lag with softsubs.
I'll ask what most likely is a stupid question, though. The torrents are properly finished when you try to play the files, right?
Incomplete files have never played at all for me, but I've heard of cases where they do.
Edit: Yeah, I get about 5-8% CPU usage from this encode as opposed to the usual ~15% from properly encoded 720p releases (for reference, CPU is a dual core i3 370M at 2.4GHz). Because really, this looks like crap and makes me wonder why they don't downscale to 480p at this point if they're going for small files... right, most people only care about resolution.
Also lol audio encoded in mono. I don't even.