I got your positive comment right here: I'm positive that you're conflicted. If you trusted this guy's abilities you wouldn't be on this board asking questions. But as it stands you like him and feel loathed to give up a guy you like. So youre trying to have it both ways.
I didn't call anyone anything. I said IF something was the case THEN he was an idget or you were cheap. I left it up to you to decide. But forget that. I'll be more affirmative. You live in a high humidity climate with undersized ducts and a furnace that literally can't push the airflow he called for. He should have set it for 400 and more likely 350 CFM. What on Earth possessed him to set it for 450 I don't know. But if everything you've said is true then he goofed. He might even be an idget. Oh the horror of my negativity!
I don't give a rat's @ss if he works 80 hours a week and co-owns the Dallas Cowboys. There are people who will tell you about their 30 years of experience and it means NOTHING because they've been doing it wrong for 30 years. Proper technique has no correlation to experience, hours worked, business ownership, how nice a guy he is or even the negative tone of my reply.