The E30 on the other hand is over 1000 pounds lighter. The S85 V10 of course adds weight to the chassis but the E30 M3 weighs roughly ~2800 pounds depending on options, trim, and if it is a US or Euro model. The V10 adds weight of course so let's say this E30 M3 is around 3050 pounds if BimmerBoost were to make an educated guess.
It is naturally aspirated so with a good tune it is around 550 horsepower depending on the exhaust setup and fuel. We do not know how aggressive the tune from Evotech is on the RS6 but C6 RS6's routinely get into the 700 horsepower range with a tune. These are not exact figures but here is how this plays out.
Evotech tuned C6 RS6: 6.32 pounds per horsepower
S85 V10 E30 M3: 5.54 pounds per horsepower
On paper the M3 has an advantage in the power to weight department. In practice, it looks even bigger than it does on paper.