Jeff Gerstmann's parents are car mechanics, I'd say he has more knowledge about cars than the average Forza player and his tastes are representative of a majority FM buyers (loves tuning very average cars to the max and try to compete in higher classes, likes custom paintjobs, tuning rims etc.). He also has tried an FFB wheel before, he drove a Chevy Nova and now has a Lexus, he was live commentator on destruction derbies and he plays racing games since the mid 80ies... I'd say he's qualified enough. When it comes to pointing out strengths and weaknesses he admits his lack of knowledge and skill where it's necessary and he smacked Driveclub for feeling archaic in it's driving model (neither arcade enough nor a good sim-feel).
But... I give you tha, when I see him try to take corners on a racetrack, I do get the one or other cringe as well, but it's nowhere near as bad as the cringes I get when I see Giant Bomb quick look a soccer game.
On the mispronunciation... check out how much stuff Darin Gangi mispronounces. "Renault Mega(h)neee" or "Rufff CTR3", yet his experience with hundreds of racing games and FFB-feel make him an expert nonetheless.
Edit: The reviews that I really dislike are the ones who review Forza just as a racing sim for their "hardcore sim" audience, totally dismissing all the features that aren't important to them, but sure are important to lots of more casual players.