A GTiR is not a sleeper.
I think people are missing the point with the idea of getting something that people dont know about (ie, Q45, Aristo, obscure imports etc) therefore they'll think its slow. Thats not the way to do it. You'll still attract cop attention, anyone who knows anything is going to see you coming a mile off. unless you're trying to trick mum's in Patrols etc. If they dont know what it is, then they will be open to expect anything. you'll stick out too much.
The key is, is to make a sleeper out of something people *think* they know about. Ie, people think Daihatsu's/Cameira (grreat suggestion!)/Volvo's are slow etc. thats where you'll get your surprise and safety from cops and would be thief's.
Also, surprised i didnt think of it. But a really good alternative to the V8 Volvo, is getting either a 960 or S90 which is the mid 90's large RWD sedan. Stock, its a 3l DOCH inline 6cyc . over-engineered like all volvos are to take a lot more than they handle. A lot of people in Sweden are putting twin turbo kits on it and are getting good results. Knowledge on how to etc, is all online and equipment available from Europe. its a common thing overseas but i've never heard of a modded one in australia getting about.