They'd have to really spin the blower up for that to be an issue, don't you think?
Anyhow, good luck.
Yeah...I doubt if the squirrel cage blower could do that. It would collapse the furnace plenum, first, since it’s square.
I can’t imagine why they are concerned about a (maybe) 20% increase in air velocity in the duct and maybe an inch of water static loss due to the higher velocity. Methinks they are just gouging customers.