The actual depiction as opposed to the description in Equilibrium is totally absurd, I think we all agree on that.
The big difference is that in real world gunfights there are so many variables that it's extremely difficult to find anything "statistically likely" enough to be able to plan ahead for it, let alone make a combat style that revolves around shooting where your enemies are most likely to be standing and dodging to where your enemies are least likely to fire.
This argument is equally valid for melee-ranged combat, that is, not at all. There are many variables yes, but most of them are extraneous. Precisely this kind of study of how the human body works and how actual fights play out is the origin of martial arts, it's totally valid to apply it to close combat involving guns. You build the style to take advantage of the predictable parts, and minimise the risk from the unpredictable parts.
At range there's actually quite a lot you can do, by learning to minimise exposure while manuvering through cover and while firing, supressive fire, tracking reload periods, etc... This is all done, we just don't normally call it martial arts, but it very much is.
Finally systematic training with gun handling itself is definitely a martial art style, such as various varieties of reloading styles, aiming, stances, cadence, etc.
Of course this also brings to mind the arguments that:
1) Gun kata would be completely worthless against untrained people, because they wouldn't be firing at the traditional spots
You can study how people with no training handle gunfights and plan for it, martial arts do this to a great extent as well. If anything, unskilled opponents tend to be more predictable, since there's no nuance to their actions, and most of them will be innefective.
2) You could counter it by aiming near them but where you don't think they are going to be. It would slow you down, true, but you only need 1-2 hits and the fight's over. (Hopefully you can get them before they hit you )
This is done in advanced martial arts, you aim an attack for where they're going to be, which may or may not be where they are now. Your accuracy at predicting this is obviously a really big deal. Advanced martial arts builds layers upon layers of feints, concealments, and misdirections in, as well as countermeasures for the above. The point about single shots being definitive is a good one, the ultimate focus of any gun based martial art is going to be in fast takedown, in stark contrast to many defensive or reactive martial arts. Defending, other than by taking cover just isn't effective.
3) Spray-and-pray would be just as effective against Gun kata wielders as normal people
Just as effective, as in hardly at all. It's actually quite difficult to hit a target at anything except point blank range without aiming. That having been said, there'd definitely be a danger zone between far enough away for spray to have a low chance of hitting and close enough to control your opponents gun. On the other hand, against unskilled opponents you just shoot them at range, because you're really good with a gun too, and your effective range is massively higher than theirs.
Forget Equilibrium though, if we have gun kata it obviously needs to be based on Grenadier ^_^
Silly gun kata aside, we are planning on having military martial arts incorporate gun handling aspects. (probably the American and Russian special forces styles, the names escape me).