Of course you must understand, I look at this from a different point of view.
The 2.8 car will be EFI, I don't remember if it is LH (separate ignition with dist cap) or HFM (integral ignition, with multiple coils waste spark ignition). Neither system has any external adjustments. They are totally self-learning, self correcting both in mixture and timing and engine idle speed. NO ADJUSTMENTS.
If spark plugs matter to the emissions of these systems, the cars most serious problem is a lack of attention from its maintenance manager. Plugs are still recommended (except in Ca.) at 15k intervals. They easily go the Ca. recommended 30k and probably much longer. I don't happen to be on the group that thinks you can improve this motor with plugs. At least not until they are just screwed.
If plugs really make a difference, it will be because they are being hurt by a condition in the cylinder - the other type of serious problem. Problems such as leaking injector, leaking valve guide seal, too much carbon from long term valve guide seal leak, and in the end compression problems could occur that plugs could help.
A proper maintained car will not have any chance of failing these tests without a problem. Plugs can only mask the problem. If there is no problem and plugs help the problem then there is a problem.....with maintenance!
Just my opinion , of course.
Bosch Master, ASE Master, L1
33 years MB technician