The way I look at this is you have to do certain mods, like brakes and tires. But pick something, and stick with it. If you keep getting stickier and stickier tire, you won't know if you're improving.
Also, THE best way to improve is to do track days at wet. Every mistake you make will be amplified. I have done couple rainy days in one of our local tracks, and I can tell the difference between myself and who haven't done any.
2011 MINI Cooper S
previous cars: E92 M3, Z4MC, Z4 Roadster, E36 328 Sedan