Jungly AeO - I just pass on what I am told by people who are as close to the Mk4 plus's as it is possible to be - I won't name names.
I also know people who were directly involved with the Qinetic trials and testing of the Carson blades fit and using a non-folding head with bifilar absorbers makes a significant difference. Take the folding head off and you will see a significant reduction in drag but the speed issue is all about vibration not drag. You don't get an increase in speed from the Carson blades fit because the Release to Service won't let you go above 127kts - (again vibration issues and fatigue) what you get is that same speed at MAUM and at high altitudes. With fixed head and bifilar absorbers you would be at 140 kts plus in a smooth aircraft.
The fact that the RN elected to continue with the folding head says much about their dogmatic approach 'it is a Navy aircraft therefore it must have a folding head for shipborne work' even if the whole aim was to produce hot and high capability for land based ops.