We run these on engines of around 500HP pretty often. I think I have had some approach 550HP but thats probably about it on a pump gas street type deal. Then you need to get more involved in porting.
One of the reasons I developed our own casting was a desire to get beyond that on a simple build. The Survival head will be a little better than these on a 445 - maybe 20-30HP, but will carry up past 600 on a 482 without going to a full porting regimen.
A CNC porting program will get either head far higher in potential. CNC work from my guy runs around $1500, and I know that other folks selling heads have a lot of Edelbrock based programs as well. These days a lot of my Edelbrock based work is an effort to save the customer some money since a lot of folks already own the castings.