Brakes.
Take a look at this video.
Listen from 0:41 to 0:51.
If you've found your driving has plateaued, one area you can make up the biggest ground in terms of improvement is in brakes. Be it better equipment or better technique, or learning to adjust your braking specifically to maximize the amount of braking done in the least amount of time.
And I agree with Mr. Kendall. Once you get up to about 9/10th in speed, the last 10th is all braking. It's the one thing that separates the men from boys in high performance driving.
And especially on this chassis, as compared to the E46 M3, there's more power per pound of mass, with the right equipment under your foot, you should be able to brake later, brake harder, but do it in a much shorter span of time while maintaining higher speed through the turns and also better acceleration out of turns. But all that advantage is wasted if you're approaching a turn at the same speed or maybe even slower because you had to brake longer since the E46 actually has better body control due to the longer chassis.