I'd agree. I imagine Luthor's voice as somewhat deep, but slow, measured, sinister at times. He's the epitome of confidence and suave charm. I never really get that sense from Cooper's voice.
I dunno, the guy has a smugness that makes him a little unlikeable at times, but I still would find it hard to buy him as a villain. I think the problem is he just doesn't exude any danger; he's more of the unlikable jock type (like his early roles) rather than someone who could actually do serious damage to our hero.
Billy Zane really does have an exceptional look for the role of Luthor, just a pity he can't act.