Hi all,
In my case, after getting some advice on this forum, I had two other mechanics do a leak down test. One of the mechanics was a name from the comments here, and one is someone I've known for many years who works on Porsches. I told both about the symptom (not requesting a leak down test), but both did that same test and both reported the same exhaust valve had failed.

So I went back to Oscar at MB Class Motors, who has been doing the work on my cars, and had him do the valve job. When the head was out, I looked it over. The one valve had a circular hole of about .25" in diameter that touched the outside circumfrence of the valve. Oscar sends the head to the same machine shop that's used by both local Mercedes dealers (Fletcher Jones and Liguna Niguel), so I felt confident in their ability. They recommended only replacing that one valve. While the other valves and the head showed a little carbon, there wasn't much and they were all within tolerance. The engine (the whole car) has run quite fine since then (just about 1 year, now). The engine is about as smooth as it ever was, and power is fine.

Why a valve job at "only" 152k miles? My (inexperienced) guess is that the hole in my exhaust valve was caused by some particle of foreign matter. Perhaps dirt in gasoline that made it through the filter, perhaps dirt that got in through the spark plug hole when the plugs were replaced at 150k miles. Others here may have better knowledge on that issue. While the head was out, we also plugged the hole that is known to lead to head gasket wear, and replaced the timing chain and tensioner. I didn't know about the 3 coils and the connector at the time. so I'd have to look at the invoice to see if Oscar replaced the connector by himself. Btw, gas mileage improved about 20% after all this work. Total cost ended up about $2400.

In the end, the car really did need the valve job, though the reason is a mystery. I was, and remain, very greatful to the members of this forum who gave me the recommendations that helped verify the problem and pointed me at a few good shops in my area. I will likely be using one or more of the shops given here for some work in the future.

__________________
Eric