I believe you're making a mistake in those calculations. You're adding cars twice. Don't forget a lot of the list we know for GT7 overlaps with that of GT Sport. Cars that are in both games shouldn't be counted twice.
I did a count just now, assuming all of GT Sport's car list makes the jump, as it is. I used Kudos Prime for the GT Sport list, and counted all cars listed without years as being 2011+. Some obvious exceptions like the classic PD Formula car were ignored. With that in mind, GT Sport contains around 220 cars from 2011 onward. Based on the list we have, here on GTPlanet, 19 cars, from 2011 onward, are new for GT7, so far. Which means, we currently stand at around 240 cars in that age bracket. We need another 60 VGTs, fictional race cars, or real modern cars to reach the 300 mark.