Rather than removing lots of almost-the-same cars, there should be a more intelligent way of counting them and organising them.
Instead of having an entry for every MX-5 in the Mazda dealership menu, maybe there should be just one entry. If selected, this sends you to another menu that lists every MX-5.
Counting cars... is a little more complicated. You can lump lots of the Nissan Skylines together, but not ALL of them, since some are modern, some are old, and some are extensively modified for racing. Same goes for the Chevrolet Corvettes, and lots of other cars. (I suspect it would be fair to lump together all the MX5s though.)
Also see this article:
http://britgamer.co.uk/2013/11/06/how-many-cars-does-gran-turismo-6-really-have/
This guy tried to count how many "distinct" cars were in GT6, and he came up with a figure of 434. While it's a respectable number, it's really far from the 1197 advertised by Polyphony. In some places he's been generous. He counted 11 "distinct" Skylines by treating each "platform" (R30, R32, etc) as different. In other places, he's been stingy. He counted only 1 Corvette, even though there's over 50 years between some of them. He also hasn't distinguished between road-going and racing variants of the same car (like the normal and JGTC Supras).