He wouldn't KO McCallum but he did school Maurice Hope with surprising ease. Benitez was a formidable opponent at 154 more so than anyone McCallum faced at the weight in my opinion.
He is too small to beat McCallum imo, McCallum was a rangy fighter who could apply pressure when needed (McCrory, Toney 2
) or beat you in a technical Boxing match.
I dont think Benitez would find sustained success outside because McCallum's reach dwarfs his, plus McCallum had a terrific jab and the ability to hurt Benitez.