Tech lowered gas pressure

After about 3 months of heating usage my Carrier 59SC2E furnace started high limit trips. Unit had been checked about 6 weeks ago, manifold gas pressure verified at 3.5PSI. I called the dealer, tech shows up, screwdriver and 2 thermometers and without use of a manometer lowered the gas pressure. When asked to verify the manifold gas pressure is within limits he flatly refused. Previously the heat cycle would run for 20 mins, now it runs for 2.5 hours and barely gets back up to desired temperature. His aim was to set temperature rise to 50-55deg. I have read the install manual, gas pressure must not be lowered below 2.8"WC. Without a manometer we have no idea what the pressure is. The install manual talked about adjusting temperature rise by raising fan speed, but I suspect that unit has an undersized return duct, being 16" round flex duct on a 3 ton unit. Is he correct in lowering gas pressure and risking furnace longevity or should he have verified it at 3.5 and increased fan speed? The company has installed hundreds of units in this development, but I suspect they are covering up the undersized duct for the potential 1200CFM flow.

Thank you.