The 1Dx high speed continuous shooting is 12fps if shutter is 1/1000 and ISO is set under 25600. JPEG lockdown mode is 14fps. I know I'm being pedantic but now you have an extra 2fps you don't need!
As for high fps, as Alex says, it's very useful when you need it, but useless when you don't
. I shoot different things than Alex, who is a master of fish behavior. I shoot predation behavior of big animals like sharks, whales etc in very fast action. One could say a good photographer would know when to pull the trigger, but for my modest talent (or lack there of), that's in the realm of "use the force, Luke" and I'd rather use the inelegant approach of attrition.
This brings up high speed flash, which allows me to lfill light animal subjects with ambient backgrounds, especially with cameras with excellent ISO performance. In baitball and other predation behavior, one could be trying to light up a 2m shark in a baitball, something the little INON would not be able to do but my big 250W Hartenberger chomps for breakfast. This ties into the higher shutter speeds which leads to the eTTL protocol. The Canon eTTL controls high speed sync, and so eTTL is essential for high speed sync.
And eTTL also ties into program modes which can also get you the shot when the action moves unpredictably. Most people will say TTL for wide angle underwater is not a good thing, and manual is preferred. Normally I agree but for certain situations, it works well to control your fill flash. It can also get a shot you aren't ready for.
That leads to strobes that can do eTTL, which only a few manufacturers can, including Seacam's SeaFlash. I did a review of them a couple of years back so you can find it in the archives. Lastly, only the SeaFlash can do rear curtain, which is also part of eTTL, so if you need that function, you'll have narrowed your choice.
Like I said before, list down the things you want to do with the housing, then choose the housing that fits your needs. Personally, I'd wait for the Seacam to show the 1Dx housing before putting money down. With the 5D3 housing, Seacam tossed out full joystick control which made the Q menu functionality limited. I personally use the Quick Menu constantly for stills. So I didn't choose the Seacam because of that. I have no idea what the Subal will look like either, since they aren't delivering yet.