I think instead of producing twenty of every car the exact same way in GT6, why don't we instead use the regional bullcrap versions as a basis for this explanation. We'll use the Nissan GT-R for this one.
First, a Japanese-spec GT-R is fundamentally different from an American-spec GT-R. When it is shipped to the US, it doesn't recieve the necessary suspension tweaks for driving on Japanese roads, therefore thesuspension is softer, and less responsive. Also, there is a slight drop in power, and if you live in California, an additional drop in power to meet emissions standards.
Fortunately, emissions regulations don't mean a damn thing in videogames, unless it's a green game. Thankfully, no such thing exists. GT6, should apply these subtle differences, which ironically enough, they've already modeled an LHD version of the GT-R. All that would be needed is the suspension adjustments written into the code and essentially mirror the RHD version of the GT-R, and replace the Speedometer with an Imperial one.
If they do this with every car that is made with steering wheel on both sides, it saves them the trouble of wasting hellacious amounts of money, doing everything from scratch.