GT6 is really accurate on stock cars reactions as long as you keep them on comfort tyres (softs if you'll go for S-tyres in real life, medium for good road tyres and hards if you're the kind of person to save some money on tyres), so to compare track performances in the game is not dumb. However, there are still a lot of parameters which can't be reflected by a game of any sort. Since you drive all cars with the same device and the same feedback, you won't be able to know the actual sensations you'll get in real life. There are also running costs, practical aspects, etc... that won't be reflected by the game. Maybe it can help to narrow down the list to fewer entries, but as for me, if I had to choose, I could as well narrow down the choices before even driving them in the game, according to what I look for in a car.
Anyway, I think the necessity to have a real test drive before buying a car is obvious. If you can try out several different cars before making a decision, that would be the best.
Your list contains very different vehicles, so you might want to think more deeply about what qualities you're looking for. Since you owned a NA and you're considering another one or the new ND, you may want to list the pros and cons the NA had in your opinion, do the same with the Megane RS you sold, and work from it. You will surely find some points you don't want to sacrifice, some you don't care that much for, and some others you would like to improve.
I myself own a NA, and it would be a real hard time for me if deemed to find something as fun and versatile as this little thing. In my budget, that was not the fastest, nor the more practical or economical choice available, but for my use and preferences, I never came across something better.