For what its worth, I was having trouble with it as well as I was trying to beat him with my Lamborghini Murcielago PI 1488 (the Scion is PI 1900), which I managed to do so relatively easily the first couple tracks, but the last couple I just couldnt quite beat him. (still won due to series points), but I had assumed it was PI B rated cars (dumb mistake).
Most tracks are best suited with fast acceleration and high speed cornering cars, as there are too few places to really break 150+ mph very often.
Since then I've gone back and used a variety of cars to beat him, but the key was definitely using a much closer rated PI car in the A range. The Acceleration aspect is relatively easy to achieve by picking the right cars and using upgrades for acceleration (top speed isnt all that imporant) and simply tuning the engine for acceleration. The cornering/handling is more tricky however, and here is where it really comes down to how you're best at driving corners yourself.
Personally I dont recommend using anything in the 2000+ PI range, as it may be easier to beat Rado, but you'll also have a much more difficult time managing the car itself, as well as much more irritating competition.
A 1600-1800 PI rated Lamborghini Murcielago should have no problems beating the Scion. Typically a clean lap while "playing it safe" was enough, and I found more success using that method than pushing hard for the most out of every corner with occasional errors.