I have driven most of what BMW has to offer around a track and I did not find that I had more fun with a heavy car with more horsepower. The 2019 M5 Competition around the track was fun but just in weight alone it did not make it more fun because I could track it faster on the straights.
The M2 on the other hand was a whole lot more fun around both tracks I used it on, in comparison. The M4 was also fun.
To my point, why build an RS6 to go for an occasional track day? I think your money would have been better invested elsewhere for a track focused car, where it is lighter and handles better.
Get the RS6 well sorted and enjoy it daily, and slam the gas pedal on occasion on some on-ramps or empty highway cruisin'...
I tried to do too much at once on the B5 S4 I had and never really got to the enjoy that car as I should have. I was determined not to make the same mistake on the RS6, or any of my other cars.