whilst working at Honda we had about 3 or 4 unmarked cop cars come threw, a couple of integra type r's which had no ariels or anything visable but had hidden buttons and sirions, and there was an s2000 as well. Most of the contracts go to aust bult cars hence why most cop cars and ford's holdens and mitsi's, most their temp cars are mitsi's when the others are at the shop. They do it to keep the profits in the Aust market which is why there are some random undercover cars but most are the standard ones.
As for the WRX's i always though it was cause they kept on killing the engines, and few people have joked at the fact that the seats are too small for the adverage police officer but the speedo issue seems a lot more belieable.
Fact of the matter is most cop cars are obvious by how they drive, i had one follow me yesterday which i assumed was a cop, changed from right lane to middle and he followed behind me, then a little later a changed into the left lane close enough in front of a car that it would be a struggle to squeze in between me and the car behind me, then the cop car quickly changed behind me, i was doing the speed limit and he followed me for ages but didn't pull me over cause i did nothing wrong.
Drive safe on the streets and you got nothing to worry about, leave it to the track i say.