Olympic water polo: advice sought

I have tickets for the some water polo matches at the London Olympics and would like to take some photos of the action. However, I have never been to a major sporting event so would like some thoughts on the feasibility of doing so.

The current policy is that SLR cameras will be allowed into venues and the plan is to restrict spectators to a combined size of a lens and camera body measuring no more than a foot long.

I intend to pair my A850 with a Sigma 70-200 f/2.8 EX APO HSM to be within the size guidelines.

My first concern is if 200mm is going to be enough reach. The tickets are category A so they should be good seats but as I’ve never been to a major event before I’ve no idea how far away the spectators are.

I do have a 70-400G but that may fall foul of the size rules and I don’t want to lug it all the way there and then be turned away. I also have a Kenko Pro300 1.4x teleconverter which may give enough reach.

Using the teleconverter will result in a stop of light loss which brings me onto my second concern: Will there be enough light for a decent shutter speed? The A850 is a great camera but it’s no high ISO monster. I'm hoping as it is being televised the lights will be brighter than is the norm for a pool but again I have no idea.

I’d appreciate some thoughts on my plan. Does it have legs or should I just give up and take my NEX-5 along for some atmospheric shots?

BTW I am familiar with the sport having played it in my youth.