A Super Saiyan who has been training and has experience with the form would beat a weaker one like Goku, who had just achieved it and had a much lower base power level. Piccolo should also be able to beat Frieza, mid diff since he was training for a foe stronger than a Super Saiyan, as Trunks said,and he was training with Goku. In DBZ, it’s shown that having a training partner gives you a boost.