Hey, I don't know if anyone has the same problem, but when I watch a video with VLC while Transmission is downloading or seeding, VLC stutters and stops playback from time to time. It gets worse with higher download/seeding speeds. When I stop Transmission everything works just fine. This was my solution at first.
But when I started experimenting with other Bittorrent clients, I found out that uTorrent didn't have any of these effects on VLC. Even with a download speed of 3MB/s while seeding at 1MB/s.
This also worked for one of my housemates who has the same Macbook Pro as I do. He even claims downloadspeeds are much faster.
So maybe if you have playback issues while downloading, try this!