The Ferrea 5000 series would be rated at heavy duty and the 6000 series are the severe ones. The 5000's would take a good beating on the street.
These are the same valves that are used in CHI heads when you buy a fully assembled set.
Street package ($3000) = Ferrea 5000 series
Competition package ($3500) = Ferrea 6000 series or equivalent (for Roller cams & high spring pressures).
Here's the Ferrea website link which explains all their valves........scroll down past the topfuel titanium ones
Not many people here may have heard of them, but they are world leaders in valve development.