[Stroked & Blown]

Thanks Randall
It looks like you saw ~ 10HP / 7TQ gain right away & it seemed to climb another 5HP as the computer learned the new breathing capabilities.
That's around 9% HP & 12% TQ gain. More than I expected. Looks like I need to do this.

Any other variables in play between runs 4/5 and 6/7/8?

Summary:
Baseline
Run 1 169.80 HP 149.56 TQ
Run 3 169.27 HP 147.91 TQ

Chinese Headers
Run 4 179.27 HP 165.85 TQ
Run 5 179.65 HP 164.38 TQ

Twin-Chamber Muffler + Chinese Headers
Run 6 181.29 HP 165.63 TQ
Run 7 182.51 HP 166.24 TQ
Run 8 184.24 HP 166.92 TQ