If you can’t change the duct, you’re stuck. Only so much volume fits. High fan speeds will just create noise and not do anything. You’re very lucky in that you have an external vent at all!

5” can’t support 600cfm. But I think that range doesn’t need a bunch ventilation. If it’s a good hood and an easy route outside you should be fine.

If you have a VaH with its squirrel cage blower, more than typically susceptible to pressure loss, the flow through a 5-inch or 6-inch duct will be fairly restricted, and adding another VaH blower to the hood may be expected to yield limited flow rate gain.

It is possible to fit an axial blower into the duct to boost the flow. However, significant boosting at that size would require a blower verging on 'screaming mimi' aerospace designs. I can assure you from experience that these are quite noisy; in their usual haunt, near jet engines and in transsonic flow, their noise performance is not an issue. https://catalog.rotron.com/catalog/large-vaneaxial-fans

I am not familiar with every existing blower, however, so a call to Fantech might yield a tolerable candidate blower that could boost your air flow rate. A control separate from that of the VaH may be required.

P.S. 600 CFM in a 5 inch duct is about 4400 ft/min, more than twice the recommended upper limit on kitchen ventilation duct velocity. Pressure loss exclusive of any other factors is about 2 inches w.c.

As @kaseki said, your issue is not the raw CFM rating but the fan curve. A good 400CFM blower could do much better for you than a cheap 650CFM blower. Fantech may well have a good solution for you.