Adding GT3 cars isn't a problem, IMO it's that a lack of career mode/objectives will naturally make online the only thing to do. Hotlapping can get boring after a while, which is probably why Shuto servers in AC are so popular.
Anecdotally, I only see people driving road cars in AC on these types of servers:
- Drift
- Cruise
- SRP
- Nordschleife
The other populated servers in my region tend to be:
- GT3's at Spa
- GT3's at Monza
- MX5's at Laguna Seca
Unfortunately, I think Evo will be a hard sell to most people. Better physics and graphics alone might not be enough to convince people to buy a new game, when the older one covers 90% of their needs.