Probably never be right unless you upsize all ducts. Even then the temperature control may be less than desirable.
I assume the HVAC tech has set the fan speed as high as possible and checked the register dampers and any duct dampers for full opening.
You should consult with the manufacturer for any relief they can offer as regulator settings or to burner configuration by plugging off some. It's never going to be right but if you're stuck with it, you'll have to accept that and take whatever measures that make it work without sacrificing safety.