Since I started driving 2 years ago, I've only been pulled over once, not counting RBT's, and that was in my old 280zx. I didn't even get pulled over in my S13 turbo, while still on my P's!
It'll often be because of the area you live in. Most of the cars around my place are Commodores with garbage-can exhausts. Much easier to defect when you don't even have to open the bonnet.
But even when I take it for a hard run, I don't get many looks. Generally speaking, if you don't drive like an ass in suburbia, you won't get any trouble, in my experience :)
As people have already said, MR2's do get mistaken for other cars, and I expect most officers of the law would freak out when you open the engine hatch and it's in the BACK. They could look in there and say "Yep, it's an engine" and leave it at that