I don't have all flash either so i personally would not trust the five seconds rpo either but it should be able to get pretty close within a minute or two, wouldn't you think.
Depends on the workload, transaction size, and the disk behind it. If EVERY application is firing off a non-hardware assisted VSS every 60 seconds you might push your poor SAN over the edge. Then again if all you care about is crash consistency and are willing to suffer a log replay then...