I'm using the 22MM pancake. Do to the tiny size of the platform we are flying on I've never heard good things about IS actually. So we have't minded not having it, and we provide a 1-Axis Stabilizer mount for it, which really just serves to keep the camera nadir on that axis. But due to no IS and the huge reduction in weight made it worth it for us.
In magic lantern we have the delay set to 1 second on autofocus, which translates to an image roughly every 3 seconds aloft (due to relatively small change in the required focus) at smooth large jpegs with autofocus on single shot mode. If the focusing has gotten better then this time could go down. If you are willing to set focus manually first you can put the camera on continuous capture before starting magic lantern and get images at maximum capture rate, but it's impossible to get it to focus in this mode. Due to a bug in the autofocus algorthim + magic lantern, when it can't solve a focus problem (gives you a red square) it will stop capturing images, and to avoid this we use the delay timer, set it to 2 minutes which is adequate time to get the plane in the air and avoid any bad focus situation. (The camera sitting an inch away from the ground can't focus in our setup). As for endurance, we get at least an hour and a half of endurance without a problem. Due to normally flying 45 minute legs, I can't tell you where dead battery will be except that it is past an hour and a half. Obviously minimizing screen brightness is important on this, as well as making sure the camera wont poweroff before the intervelometer starts.
I didn't know about that on the firmware. That's good to know actually, as we are thinking about getting another one. We swapped from a higher resolution point and shoot to the EOS M and have been getting much better results due to the change in sensor size and the glass, both predictable results.
As far as external shuttering options go, I'm not aware of being able to connect it directly to the camera like that. It's possible I missed that, I never saw any mention of support for external shuttering, and before ordering the camera I had already settled on the ML solution and didn't investigate further (except checking out tethering which doesn't happen).