Nat67
Rockgoblin
Try JHS dyno , just outside of Bristol on the bath road . He specialises in Triumph tuning and is where I take mine !Find it hard to believe someone would build an exhaust system that would lower the power? Seems crazy that it could be more restrictive than the standard set-up.
I have the full system which I would assume flowed better than the original headers but have no way of knowing as no tuners close to me and there doesn’t seem to be any suitable tunes in resources for an early Rocket.