Concur on the statement that the vac pump does not work as hard when the vacuum system is tight. Saves wear on the pump.

With regard to timing chain wear/replacement, I just measured my stretch using the lift method and had 4 degrees stretch at 270k miles on my 617. I have complete records of oil changes every 4-6k miles since the car was new. I installed an offset key to bring the cam timing back to spec. Made a big difference in performance.

Mercedes Benz does not have a specified replacement interval for the chain. They would not have made the offset keys (up to 7 degrees, IIRC) if they wanted the chain replaced before that stretch was achieved.

I could use a new tensioner rail, however. On the project list.