Ryan, You need to do your research a bit better before posting. The LX-5 and the GH-2 don't work that way. They have a special larger sensor that actually increases the field of view.
Specifically, with the LX-5, here are the image sizes compared to a S95:
4 x 3:
S95: 3648 x 2736
LX-5: 3648 x 2736
16 x 9:
S95: 3648 x 2048
Lx-5: 3968 x 2232
It actually would make the corners worse. The corner issue in 4 x 3 with a flat port, at it's widest appears to be identical to what it is on land. I believe the lens distortion is being corrected with software in the camera.
Those are the only two camera's made where that is the case. Why Panasonic goes to all the trouble to make these two sensors is a bit unclear. The Oly seems to have the same sensor, but does not use the whole sensor (also very odd).
One still looses a bit, but at least one gets something in return.
I have not tested this with add on lens, need to go over to the pool and do some test shots, but expect this will cause some problems. Might be able to do that this weekend.
I've done comparison shots, and it recovers just under half of the field of view lost by using a flat port, and given the 24mm equv., it is not far different from a 28mm equv. under a dome. Obviously not a wide as a 24 under a dome.
Your post is accurate for every other camera.
Shooting 16:9 is merely cropping strips off the top and bottom of the native 4:3 sensor. It just so happens that would also chop off the worst corner performance.
Personally I'd shoot 4:3, capturing everything I could, then decide on a 3:2, 4:3, 16:9, or square crop when I'm sitting at my comfy chair with my big monitor breathing through my nose.
See the post from Panda in this thread for comparison between a flat port and dome:
wide angle wet lens for 24mm - Wetpixel :: Underwater Photography Forums
Fortunately one can have both on the same dive in one upcoming solution.