Fit for purpose is a grey area, he asked for a car and got a car yes but do the parts that were supposed to be upgraded make the car dangerous or just not what you've paid for? The fraud lies in the car spec. I would be gaining gearbox spec serial numbers, diff numbers shaft etc and build yourself a case. Whoever now owns leasing rights to the car needs to be contacted and notified of these problems, even if they are willing to do nowt. Just to have it recorded for case.
At this point of time I would see this car as being "modified" and should be very cautious with your insurance as well. Just because Subaru rebuilt it doesnt mean its standard spec. To an insurance company, the specified parts for a WRX are'nt in your car therefore the lesser parts in place make it higher risk which I'm sure your insurance would like to know about.
My personal advice is go all out and make a huge stink about this. Most people will say it, but never follow through. Threaten media action. No car company, especially now needs a "quality control" scare over their brand ESPECIALLY one which may prove so damaging.
Good luck guys.