re: I have an outdoor kitchen exhaust problem.

Try calling Cayards in BR. (Restaurant supply company)
They should be able to give you a good recommendation.

Maybe call your contractor and try changing out the duct first, then see how it goes.
I would tell him it is not working and maybe see if he will replace the duct as part of your warranty.

If that does not work, then look at changing the fan and adding the capture hood.