GH,
Have you seen if this happens only in the real world on the road, and not on a chassis Dyno (providing you have access to one).
You may want to look into this -- Some of the aftermarket air-boxes like the JLT's are known for causing big time coasting-decel surge while clutch is depressed, and just kinda goes away as you slow down and get just under ruffly 15-20mph.
Some of these boxes cause the MAF to become very sensitive to wind, ram-air, and turbulence once the vehicle starts moving well over about 20mph. JLT's actualy started coming with a diffuser a couple years back to install in them in order to cut down on the ram-air effect that becomes much more sensitive to the MAF on those when upgrading to the larger and higher flowing Air-boxes and etc. Not sure why, but some will do it very badly and others non at all with similar builds.
Iv'e chased a ton of this kind of crap... Won't do it on the Dyno (no substantial wind obliviously), customer takes the car,, and then 1/2 hour later customer calls with an idle issue when clutch depressed on coasting decel. As long as there is some load on the motor,, there is never a problem, its decel neutral ram-air effect.
Hope this helps.