Don't get me wrong, I'm not saying that it the end-all definitive answer. Just one data point. Nor do I pretend that there aren't factors that could throw this off.
It does jive with my limited experience, though. My current build is pretty close to to a 300hp crate motor and it did fall off after 4500 rpm. Add I've seen multiple builds that swapped a bunch of parts and were disappointed with the return. To paraphrase "feels like a caged lion down low but a kitty cat up high", like someone is pull timing as timing increased.
I wonder how the cams compare between the stock 5.9 and the 300hp SBC.
