I had a Sparco Pro2000Plus racing seat installed this week by my local shop (Werks2 Motorsports in Burbank, CA). I've attended 8 DE days since January and the stock seat just doesn't provide enough support on the track.
I'm using the stock seatbelt for now and I'm of average height so I can still pass the broomstick test. But next month I plan to add the Brey-Krause roll bar extension, 6-point harness, and a HANS so I have a complete track setup.
Since this is my daily driver, the Sparco setup is made to swap in/out with the stock seat - but if I'm too lazy I can also just leave the Sparco in the car.
Very impressed with how much better the seat holds me in the car on the street. Next DE is at Autoclub Speedway (Fontana, CA) in two weeks when I'll really be able to see how well it works.