The current-generation (ND2) Mazda MX-5 is maybe the best value for money when it comes to driving fun. This is due to its low weight, naturally aspirated engine, good manual transmission and track-worthy suspension. Grassroots Motorsports have one among their project cars. In a recent article, they have listed the modifications they have made to the car and the associated gains in lap time. The goal was not only to improve the car for the track (according to what is legal in the class it is raced in), but to also keep it usable as road car.
A brief summary of the modifications can be found in this video: