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

Yeah, but the valves also have gravity on their side. I think some of yall are over thinkin this stuff. There's just no way in hell I'd run a hollow valve. I think it's splittin hairs and it's best to look elsewhere for "upgrades".

Initially the cam moves the valve, but after it starts moving you need spring to control the inertia and control bounce. Lighter valves
Have less inertia, kinda like lighter weight pistons are easier on your rods and bolts.
Bingo Duane

On valves. The valve is held onto spring with the locks and retainers making it part of the assembly as a whole.
The weight of the parts must be controlled to prevent them from doing what you don’t want them to do. You must consider all the parts as 1 part and all of its weight combined as 1 part.

The spring controls the valve & It’s combined weight with the retainer and lock.

In this scenario for an example, let’s say you have to dumb bells. The small on is 5 lbs. pick it up and hold it over your head, arm fully extended, then shake it back and forth. As fast as you can within a total swing area of 1 foot fore and aft.

Now try I again with 50lbs.

Same idea with the valve, retainer and locks.

To answer the OP’s question, I’ll start be saying, “Good question!” I’ll take a stab at it by saying, once you see/discover a short coming.
OK, that was easier to say than discovered.

I’ll take a second stab at it and suggest a lighter valve would be good or just better if your building a very hot street machine or bracket racer that see action every weekend.
This would help in the longevity of the spring and help in a quicker reving engine.

What exactly are you building this engine for?
Is the cost worth it to you?