Valvetrain Geometry
Take a look at the picture below. These are some different rocker shaft hold down caps. The two on the top are steel. The two on the bottom are aluminum. In my opinion the caps need to be steel and they need to be stout. A rigid cap will reduce the effective length of the shaft between centers. I can't see the aluminum helping very much. Even though the rocker stand under the shaft is aluminum, a wide, stout steel cap should add rigidity to the system. It would also change the resonant frequency of the rocker shaft. But just like in the example you posted with the steel vs Ti spring retainers, you never know if the frequency change will help or hurt. Rigidity seems like a good thing to me though.
I'm having some of the same thoughts, especially when that 5/16 bolt goes into an aluminum head. Edelbrock and Speedmaster use a helicoil insert in the rocker shaft bolt holes. ProMaxx, AFR and Trick Flow do not. So with the Edelbrock and Speedmaster the tapped hole in the aluminum is probably 3/8". The helicoil reduces it to 5/16"
Take a look at the picture below. These are some different rocker shaft hold down caps. The two on the top are steel. The two on the bottom are aluminum. In my opinion the caps need to be steel and they need to be stout. A rigid cap will reduce the effective length of the shaft between centers. I can't see the aluminum helping very much. Even though the rocker stand under the shaft is aluminum, a wide, stout steel cap should add rigidity to the system. It would also change the resonant frequency of the rocker shaft. But just like in the example you posted with the steel vs Ti spring retainers, you never know if the frequency change will help or hurt. Rigidity seems like a good thing to me though.