The spirit bomb is very simple to understand. We can all come up with speculations, but its a fact that in DBZ Goku never attempted to create a spirit bomb while in his SuperSayian form (To the best of my knowledge). As for the Kid Buu incident, I believe that the spirit bomb was thrown at Buu. Then when Goku had all his power wished back, which wasn't fair to Majin Buu, Goku took control of the spirit bomb. Thus I think that the spirit bomb in ESF could be changed. I won't sugguest anything now, its nice the way it is and besides I'd have to state my sugguestion in the sugguestion Thread.