The number 1 concern in previous posts was whether there was back EMF protection on the GF fans if they were overrev’ed by an external booster. The answer is they are protected:
@chris1 spent invested quite a bit of time on his exhaust system, he might know the range of booster fans used by others.
I don’t think anyone has gone up to 660 CFM. That’s probably why nobody has responded. The real issue is how much airflow is being produced due to flow resistance in the ducting. Nobody would suggest putting a 660 CFM fan right on the exhaust port with no ducting on the exhaust side, but if there’s 20 feet of flexible duct your fan is at the end of then you might be in the 'this will probably be ok" range.
The biggest CFM being used that I could find quickly is 440CFM:
Do a search using ‘cfm’ as the search term. You’ll learn more than most people will have remembered at this point. And then make your own decision.