At what point would you step up to ferrea hollow stem valves

In the context of big blocks with 2.20 diameter, 11/32" stem valves at five and a half inch long there is most certainly an RPM guideline for switching to Ti valves. Call Victory, Ferrea, a cam grinder etc. and they will tell you the same. Inertia does not rise in a linear fashion to rpm and the momentum of the valve and required force to control it's motion (ie the spring) increases exponentially. There is a reason why motorcycle engines use Ti valves....because they rev.

Take an engine that makes peak power at 7500. Stab some Ti intake valves in it and watch the powerband carry 2-300 RPM higher.