I asked about that so many times
Those carbs really are trouble aren't they? Especially if they've had fuel sitting in them eating the carb body away for a good long time. I never got a solid answer. I've heard of a lot of people replace the KZ400's carbs, but not anything but a different CV (constant velocity - read--they have those diaphrams that are very fragile). I know there was one guy selling a KZ400 motor with a Yamaha XS400 carburators. The KZ440 has a very similar carb design, but yea it is different, and yea it is still CV. It's a great performance mod, but I wouldn't expect that it would make your like ANY easier
Basically speaking, as you move away from the stock setup, your life will only get harder and more expensive. Unless you got a bunch of carbs just lying around at a friends house and don't mind screwing around with it a lot.
I personally would put money on torn carb diaphrams causing SERIOUS performance detriment and possibly even engine damage.
The good news is that the 400 is only a twin, and finding decent parts carburators shouldn't be a problem. I bet you could get 4 parts carbs (2 sets of carbs) for less than 125 US, and I'm willing to bet that at least two of them would have good diaphrams. Also check any junk yards in your area, of course.
Here is a trick I used on my 400, I coated the diaphrams with wheel bearing grease to make the material more flexible. That way, I never had any trouble with my diaphrams.