Based on photos taken by people in attendance, the car we're going to get in GT7 appears very similar to the Super Taikyu car, just with a much bigger wing than the SuTai car. I don't know if the SuTai car and the Mazda3 Gr.4 will be the same car; I'm finding it very difficult to imagine a diesel car being in Gr.4, much less be competitive—the Atenza Gr.4 is garbage even with a peaky engine. Maybe with its much wider stance and pronounced aero, it can find a place in Gr.4 à la the Mégane Trophy '11.
Mazda has always been extremely weak in Gr.4. Personally, I don't think the Mazda3 will change that status quo. I'd much rather have a RX-Vision GT4, but the 3 is still very cool to have as a fan of the brand.