The BBC has succeeded in an FOI request to VOSA to publish MOT failure rates.
Full story here
Obviously lots of interesting analysis to be done, but as a first pass in looking at 916s, I selected just the data since 1996 (as we're looking at 916s only here).
GTVs have an average MOT failure rate of 43.4%, peaking at 50.4% for 1996 reg.
Spiders have an average MOT failure rate of 39.6%, peaking at 47.8% for 1996 reg.
It's not easy to work out how this compares against other cars, though the BBC report does have a handy comparator for cars registered in 1997, and GTVs/Spiders from 1997 fall somewhere in the middle of that table.
At a guess, I'd say that the discrepancy between GTVs and Spiders might be due to Spider owners being more likely to treat them as a summer car and doing less mileage per year than a typical GTV owner.
Causes of failure (or advisory), from the most popular down (averaged for both vehicle types) are:
Lighting & signalling 25%
Fuel & exhaust 14%
Drivers view 12% (covers wiper blades, washers, and cracks/chips)