Uhm, not that I'm questioning your mechanic's knowledge but I wonder how he can state it's the tensioner to blame without any visual inspection. Starting from the left bank leakage, assuming it has been caused by the tensioner fault is pure speculation. Maybe your engine suffered a bad cam cover gasket as a stand alone issue with no correlation with the tensioner issue.
The M96.20 (and others) uses tensioner composed by metal polymer structure. In case of fail of the sliding surface you could suffer a lot of secondary effects as oil pump failure, oil pressure drop or lack of lubrication in a specific area of the engine aside from a huge variation in the timing of the bank if the tensioner cannot correct the loosen chain with its own spring effect. Don't forget that this will put a lot of plastic debris in your oil circuit that you should see in the filter cartridge.
My boxster sounds to be very noising at start up until the oil reaches its correct pressure and flow inside the engine (oil grade to be adjusted, now I'm using 0-50W). And that noise is coming from chain tensioner which operates without oil for the very first revolutions. Then you ear the engine running smooth and silky. In case of a bad tensioner, a noise would be audible also during normal operation.
Don't know... I am convinced that an engine should be swapped only if there's not any other chance. Oviously, imho. Let us know
