Seems all is well on my Diff and props haft. BUT.... Seems like the engine is making a whole lotta noise and getting nowhere

Technician took her for a spin and reckons all is good with the drive train, but that she's dreadfully underpowered. Seems that either the air inlet sensor is faulty and or the cat is getting blocked. Also talks about a resonance in the exhaust... I can't hear it....
Looking at these threads: https://www.pajeroclub.co.za/forum/view ... ter#p32252 and https://www.pajeroclub.co.za/forum/view ... ter#p29448 and https://www.pajeroclub.co.za/forum/view ... ter#p27387 what is the best to do???
Technician says that a free flow will be problematic (increased fuel consumption) due to the before and after sensors will give a reading that will tell the engine to use more fuel

What say you guys out there?