I was standing at a fight night (Muay Thai) next to someone with Canon 5Dii with grip and 24-70mm/f2.8 lens. He was complaining about his camera, because it was so large and heavy. Even having trubble with his shoulder since ringfights was his main way of sportsphotography (while I'm there every once in a wile). My set up was two camera's, K-5 with FA31mm and K-5 with DA*55mm, changing camera quickly when needed.
There is a market for a full frame camera that is small (as possible). I would want that.
My picture got in a paper, so there are options enough to improve and get better to FF starting from a K-5 styled camera.
Fightpicture