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What made you deduce that the decel valve is bad? The reason I ask is that the decel valve does not bypass any air around the throttle at idle. If it does, then it is broken. It also has no emissions function. It's only job is to limit the maximum vacuum during decels, hence its name. To make this function more clear, they renamed it to vacuum limiting valve for the C2 turbos. As has been mentioned, it keeps the compressor from surging during shifts. Without it, you will hear what is described as a horse whinny sound, which is the noise your compressor makes when it is surging, during a hard decel.
You should test it by pulling a vacuum of 20" Hg on the vacuum hose connection to see if it will hold it. If so, the diaphram is good. If not, it is toast. If the diaphram is good, but the bypass is leaking, you can sometimes fix that with a swab and some carb cleaner while applying vacuum to the hose connection.
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