82 GPZ550 --> 615 RPM hang

I've got an '82 Gpz550 that I've upgraded with a Dynoman 615 kit and a MAC 4-1. Last summer, I was playing with TM-27 carbs, but they wouldn't idle right. Start easy, but give it any gas and the RPM would shoot to 4500-5000 and hang. I ran through every size of pilot jet that Mikuni makes, and while I could smell the mix get richer, the behavior was absolutely identical in every case. srsupertrap was extremely helpful in this process, but I never solved it.
Here's the thread: www.kzrider.com/forum/forum-index/3-carb...50-mikuni-flatslides  

I've put the stock carbs and airbox back on (with stock jets right now), and the behavior is *exactly* the same. Starts with some choke, is able to idle. Give it any gas, and it'll shoot to 4500-5000 rpm and hang there. If I put it in gear and drag the clutch, it'll bring the RPMs back down.

This seems like an air leak, but I really, really don't think this is an air leak. Boots on the head and airbox are all new. The airbox is intact, and I've sealed up the holes. What's more, I tried without a filter just to see (simulating a big airbox leak) and the behavior was *exactly* the same. I also put the filter back, duct taped over half the intake port (which should make it richer) and the behavior was *exactly* the same.
The other thing is that it seems very weird that two sets of completely different carbs would have such identical behavior.

I sprayed WD-40 all over. I tried propane. Nothing indicated an air leak.
Same behavior with throttle cable completely disconnected.
I've turned the mixture screws from fully closed to 4 turns out. The idle will smell richer and get a little rougher, but the behavior will be the same.
I have checked and re-checked valve clearance.
I have checked that the carbs are balanced.
I have checked the spark advance with a timing light, and it's a brand-new advancer unit. Moves smoothly by hand, and I can see it expand and contract with RPM.

One small possible clue is that the idle speed screw doesn't have much effect. When the idle is "normal", it takes like a quarter turn to change the idle speed noticeably. Giving it gas causes the exact same behavior, regardless of idle speed screw though.

What else could I possibly be missing?

I do remember and have just re-read your other post.  I am not the expert with this carb, but they do interest me.  It sounds like you have done most things I would try without success.  I would say it makes me think it is not a carb/intake issue, since it does the same thing with stock carbs/air box.  Though it still could be that the correct jetting/settings on both set of carbs has not been found yet. 

Do not take any of my suggestions in the wrong way.  When one does things that should solve an issue but do not, then one needs to try something else for a solution.  So things I can think of that do not involve the intake system (fuel/air),  are throttle cables could they be binding or not set correctly (you did mention that the idle screw on the cable does not have much movement before there is a big change), advance unit not that it moves but that it moves at the correct point, if equipped with the clean air system the reed valves on it and or vacuum ports that were part of the system, valves (which you double checked I know)