In addition to the known issue of leaking seals, the original Hydraulic-style de-tensioners do not work very well in extreme cold temperatures. If it's cold enough to prevent proper oil flow, there will not be sufficient oil pressure to feed the de-tensioner. I know this is an esoteric failure point, but it's one that Tom Zat told me played a significant role in the development of the original Staybelt tensioner.
I might be missing something, but I am not sure how this would be an issue. The Busso de-tensioner design keeps the belt fully tensioned for the first few seconds while cranking a cold engine, which is good because that is when most timing belt failures happen. If the ambient temperature is below freezing and it takes a bit longer for sufficient oil pressure to build for de-tensioning, how would that be a problem?
I think the over-complicated Busso de-tensioner design was deemed necessary because of the perilously small amount of belt wrap around the bank 1 camshaft gear due to the adjacent auxiliary shaft pulley. Any other T-belt-equipped engine I have seen has had full belt wrap around both cam pulleys (including the later Alfa 24V V6, which uses a conventional, spring loaded tensioner).
In any case, I have one of the new Staybelt tensioners on the shelf and ready to install during my next timing belt/water pump replacement job. My GTV6 engine had the original de-tensioner operating as a fixed mechanical tensioner when I first bought the car. I opened up the epoxied oil feed stud and rebuilt the de-tensioner using the best parts from two different used tensioner units. It leaked oil, so I re-sealed it again using the good seal kit from Alfissimo, but it still leaks a bit from the shaft seal.
As a Porsche mechanic with a lot of experience with 944 and 928 timing belt systems, I must say the early 944 with mechanical timing belt tensioner works quite well as long as the factory instructions and replacement intervals are followed. At my shop we re-tension any new 944 timing belt (mechanical or later spring-loaded style) after 1,000 miles to account for belt stretch. This would be a wise idea with any mechanical belt tensioner.
Anyway, thanks to Centerline for continuing to support these cars!