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

Yeah, what RRR said in post 21. A sodium valve is a hollow valve, & the strength is compromised 'cause it's got a hole in it.
These valves were originally designed for extreme/high temp operating conditions. Like an aircraft engine running hour after hour....
Never designed for the huge spring forces of today's high rpm, roller cammed engines. So good luck with a weakened valve....