Their ad says "Provides maximum flow rates". I don't think it restricts like you are speculating. I wouldn't worry much about it, unless you are in a hurricane region, then it could overtop regardless, in a storm like last year near Houston.
Yes, I called them to see if they had done any testing and they said they had not.... so hopefully you're right and I am overcompensating for what I think the restriction would be. I know when I researched the flow rate on a 15" pipe vs a 24" pipe it was like 7,000 GPM vs 21,000 GPM...possible flow rates if all things were optimal.
