Glad you were able to decide on a solution. The thing to keep in mind with 3rd party TCs is possible incompatibility esp. if you use AFMA. You might find yourself picking up a TC in the future to use with your 5D/70-200. Are you using a monopod/tripod? I've tried an EOS-M + 70-200 + 2x TC and can get unweildy without support.
Thanks for the tip! I was just gonna rely on the IS. A monopod would be a great idea! I don't have one but I think I might be able to borrow one from a friend. The event is in a baseball field with benches and chairs. It's it a sort of fireworks display with these special giant handheld roman candles. Light levels will be low but I want to capture some of the sparks falling so I'll prob have to drag the shutter a bit.
The M is just a backup. I'll be shooting mainly with the FF, cropping in post if needs be. If there's time I'll make the switch to the M and get a couple of tight shots of just one person holding the firework. It will also help me decide if I should think about investing in a 300mm lens or not. (Any excuse to buy gear, eh?)