A 5.5L engine costs like 4.000$ per year in taxes here in Belgium, and probably other European countries.
A 3.2 L would costs about 1.200$ per year in taxes.
Just for that, I'd have M3 over C63 AMG any day.
You Americans have the luck not having to worry about engine displacement. We do. And I guess BMW takes this into account as well when designing new cars.
And don't forget that 6-cylinder engines is what defines a great part of BMW's legacy, including multiple M3 generations.