UPDATE:
My car has been running very well lately (knock on passenger wood footwell). It's hot so I decided to revisit the detonation I was getting with the FR Wilk chip. I plugged the "jumper" back into the atmospheric pressure plug on the ECU and went for a drive. Detonation is gone!
I've done three things since I last tried it:
1. Belts were retensioned by mechanic to post-break-in specs. Mechanic noticed timing belt was loose.
2. Ran through a tank of Lucas Injector Cleaner
3. Per mechanic's advice, waited until the oil was warm and drove the car hard for 1 hour -- high RPM and high-load scenarios while using the tank of gas with injector cleaner.
I asked my mechanic about sending out the injectors for cleaning and he said he thinks in most cases it's a waste of money -- injector cleaner + hard driving every six months will keep them clean on the Motronic cars.
I guess my injectors must have been dirty. As I said, I've noticed the car running generally better (more power and the motor just seems happier) and that's what prompted me to go back to the FR Wilk chip and try it again.