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OK, there's alot of questions here:
BeepBeep - "Which cars open idle valve on deccel?" An example that I have seen recently is the 2004 Dodge SRT4. After '04 most cars switched to ETC. There is no more IAC. In current cars the throttle is held a little further open with the ETC to produce the same effect. The 2004 Porsche Turbo is one that I have seen in that category.
For modern cars, the ability to shut off the fuel enables the IAC or ETC to be open further. Otherwise you get a big RPM flare when you add air to all that fuel. That is the CIS "lazy throttle" effect. Keeping the manifold vacuum from getting too low is good. When you get back into it there is less time needed to build boost back up.
IAC's are too slow? I don't think so. Maybe the big old Bosch rotary ones on the original Motronic? I don't know about those. I have a GM IAC on my EFI conversion. I "clocked" it tonight. To go from 10 steps to 150 steps takes only 0.55 seconds. More modern LIAC's like the later Motronic systems are even faster, and ETC is faster still.
This is way off track from the decel valve issue though. To summarize:
Decel valve is good because it limits manifold vacuum. A high vacuum manifold is an empty manifold.
Decel valve is good because it can dump some of the pressure spike during shifts before the bypass system can react.
Decel valve is good because it burns the fuel injected by the CIS, which some claim might help turbo response. We know for sure that exploding it in the muffler certainly doesn't.
Decel valve is bad because it can fail, and fixing it costs money, maybe more than it is worth.
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