Goku has to sustain certain energy reserves to be capable of achieving his super saiyan levels. granted he can basically stay super saiyan with no attempt, however that would not be enough to challenge superman. Goku also loses power and speed capabilities the more he is beaten and the more fatigued he gets. Yes it takes alot for him to get this drained,but it happens. I am more than certain superman would be fast enough to keep Goku's energy blasts at bay. If not it wouldnt effect him that much. It would take a long consistant pounding of his top energy blasts. Goku doesnt have the abilityto keep pumping out the strongest of his artilary without tiring and having to bust down a saiyan level. It takes Goku great concentration and a large amount of his energy to pull off a kahmeha what ever the hell it is.when you knock superman down, he is going to come back as strong as he was when you knocked him down, if not stronger. you knock down Goku enough, and force him to use his energy he will eventually be nothing more than a super saiyan or just a saiyan.