I'd say, many of the original courses are WAY too dangerous for a real-life circuit. In El Capitan, you'd have to hope you won't land in the middle of the Yosemite rallys... In Trial Mountain, you'll hope there won't be a cliff named after you, complete with parts of you all over that cliff. Those, as well as others, too, are all too dangerous. The high-speed section in the rear side of the mountain in Costa di Amalfie, as well as the whole Citta di Aria course, would be pretty unsafe (especially those bits of walls sticking out of the Citta... it'd smash any suspension & front bumpers).
Among the safe ones are Autumn Ring, Apricot Hill, High Speed Ring (the banking, plus, you really barely have to brake, even on cars like the MB CLK-GTR).
Although, consider Seattle, New York, Hong Kong, and the likes as street-racing courses, where really high speeds wouldn't be possible - since when car street-legal cars get fast enough to fly THIS high, and, in real life, there wouldn't be such sharp jumps - it'd all be a smooth angle, like there is everywhere. I guess it's the polygon-count that made developers of games make such sharp "jumps" on upwards-going streets (San Fierro from GTA: San Andreas, too)