Not so much the rocker shafts as reducing oil temps in the head and as it drains down to the followers/cam. Based on Jim Comnoz’ scar testing it appears that many oils are OK up until a critical temperature is reached and then, higher than that point, the oils fail and damage to the cam or followers is possible. Further, the head, exhaust valve spring pockets especially, is a likely place where such damage occurs and where pre-cooling might be most helpful. As we know, oil temps at the tank are generally appropriate and usually need no reduction. Oil to the head, however, gets pre-heated as it enters the engine and then goes through compression heating at the oil pump before it goes up to one of the hottest parts of the engine.
Whether there is enough volume of oil being delivered by way of the rocker lines to effect any meaningful temperature reduction is the real question. Stock nylon lines were 3/16 IIRC and the braided stainless kits (at least the set on my bike) are apparently AN3, which while nominally also 3/16, chokes down to less than 1/8 at each of the banjos' barbs. So, since small diameters reduce flow, I'm looking at replacing my lines with 1/4 inch, or AN4 so that a cooler has more of a chance to be helpful. And then, that assumes that timing chest pressure regulator will allow that greater flow.