I have just spent about 1 hr. reading over this thread, I spend most of my on line time at the 300c forum and have experienced this degragradation due to normal driving. The origional question was about the adaptives (not warranty) and if they affect canned diablo tunes. I still don't think there has been an acceptable simple answer. Does the tune change the parameters/tables that the algorithms use. If they do it seems to me that the adaptives will degrade the "tune" and would need to be re-installed.
I personally am wondering if what you guys are feeling, is the car after it learned it's ltft's. And possibly if you had just become used to it.
Re-installing the tune possibly wipes out the learned fuel trims, and could cause the car to run differently until they were learned again.
I believe there is a fuse you can pull that will wipe those trims without you having to remove and reload the tune. Maybe try that and see if you have the same experience