An early Mk2 A3 3.2 quattro Sport (ie not in gussied-up S-Line form) is also a very meek looking little thing that when debadged is totally indistinguishable from a cooking 2.0 Sport, yet kicks out a healthy 250bhp.
I think the family of volvo t-5 estates does a pretty good job of being stealthy, and after all, plod doesnt use them for nothing. Yes i accept that rs4 etc etc are also good stealthy cars, but i wouldn't mind seeing that passat/rs4 hybrid that JAYB was on about