lol! Guilty of the Borat-ness, too.
We have a Corvette, and I notice other Corvette drivers who I pass on the road do this weird "one-handed-wave-from-the-steeling-wheel" thing. No more..no less. Its kind of odd, I guess.
When I want to compliment a car I see on the road, I wave & sort of point to the car and compliment them, but I dont always think the driver knows what I'm doing. I'm sure I look
completely insane.
Once, the driver of this awesome Camaro near where I live followed me home after I saw him at a red light & complimented his car. I guess I made a big deal about it. When I walked down my driveway to see what he wanted, he asked for my phone number.
(my husband told him I wasn't allowed to give it out) 
Since then, I kind of play it a little cooler.
I'm not sure there's a cool way to tell someone on the road you like their car, I sure haven't found a way of doing it yet.