I've got an answer regarding the dip in the torque curve, here it is:-
Hi John
The dip is a mechanical issue whether it be exhaust tuning, engine characteristics or intake anomaly all can cause a dip in both power and torque and is unaffected by fuelling.
As the CAT link pipe has emphasised the dip, the root of my attention would be toward the exhaust. The link pipe may be causing a pressure build at a certain frequency either hindering scavenging or restricting the gases. Playing around with the exhaust and using dynometer evidence to show variants is the best option to find a solution.
Kind regards Jon