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you have two types of maintenance - reactive (replace when broken) and preventative (replace before it breaks). reactive - spilled some milk? clean it up. grass getting long? mow it. on the other hand, preventative maintenance is done to reduce or defer a potential failure that would cost lots more to address as reactive maintenance further down the road (ie, you paint your house every 5 years to avoid having to replace the siding every 10).
vehicle maintenance schedules are developed the same way. tires worn? get new tires. brakes or shocks shot? replace them. all reactive maintenance. bad oil or worn timing belt can grenade an engine, so these are replaced as a preventative measure.
the m96 engine was new to porsche when it came out, and they didn't quite know what would wear and what wouldn't. as a result there were some 'misses'; items that we now know do wear out, that their failure has catastrophic consequences on the engine (ie, reactive maintenance is too late or VERY expensive) but are not listed in the maintenance manual as preventative. ims, water pump, chain tensioners. these should be preventative maintenance items but are not listed as such by porsche (probably due to the liability associated with acknowledging these issues).
but it is not black and white. no-one will tell you that your ims WILL fail at 96k miles, that your water pump WILL fail after three years. only that it MAY. you need lots of data to make accurate predictions in this field; porsche appears to be holding theirs close to their chest, and anything you get on the internet is worth what you pay for it. i would note that porsche has been constantly updating (to the point of eliminating) the ims, so that tells me that they are very aware of it.
so, in the absence of qualitative or quantitative info, you have to make your own call; defer a small cost with 'x' % odds that you invite a large cost in the future, or fold your hand and replace the damned thing when you do a low temp tstat.
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