Welcome to the Forum!
Could you possibly post some pics? We love pics!
I'm not real familiar with that lens, but could it just be slow to AF as opposed to the 3.4fps being a problem? If the lens is slow to AF, upgrading your camera won't really address the problem.
I have a 7D that will shoot 8fps, but still shoot sports in low-speed continuous mode (3fps) most of the time. I usually shoot with Canon L lenses, so AF speed is not a lens issue for me.
The AF speed on the Canon 70-200 group of lenses is excellent. I used to own a 70-200 f/2.8 and I understand the newer versions are even faster, but would 200mm be long enough for surfing shots?
Most of the surfing shots I've seen are in the 300+ range with many taken at 400mm (probably because the 400mm f/5.6 prime and the 100-400L are the end of the less expensive, long telephoto L lenses for Canon).
Before purchasing a 70-200 f/4L, I think I'd try borrowing or renting one to see if that range is acceptable. It would also allow you to see if the AF speed of your Sigma lens could be the problem.
This could give you better data on which to base your choice of actions.