http://www.arp-bolts.com/pages/tech/matspec.html
The ARP alloys can all be mapped to standard commercial alloys or modifications of commercial alloys. The Russians apparently have a "super" multiphase alloy containing some aluminum developed during the cold war; some tests have been done on it (it's good stuff) but we haven't pursued it as they have now patented it and may be producing it commercially.
The best available overall stuff for head studs is probably ARP3.5 (MP35N); it is hard to beat for a combination of strength and corrosion resistance. It is likely an overkill choice though given it's high cost (35% cobalt content). The 17-4PH while just barely a space age alloy (there were V-2's being launched when it was introduced to the aircraft industry after WWII) is a good choice for head studs but only because this application is less demanding strength wise as the Porsche engine case base metal (Mg or Al) will likely fail first even with thread inserts.
As an aside, 17-4 PH would not be a good choice for rod bolts (it's too weak) and I don't think you'll find anyone using it for such an application.
Cheers, Jim