If you don't drive in the narrow RPM range that the primaries tune to, or your cam doesn't have enough overlap for extraction, the next best feature is the largest exhaust valve and header you can fit to reduce pumping losses.
Major work was done on tuning L6 motors 80 years ago, and keeping the 2 groups separate all the way to the atmosphere has been the most successful for a road car: Jaguar, Aston-Martin, Healey, Zodiac, etc.
