Piccolo has been constantly training and has stated on multiple occasions that he never stops, but by having Kami and Nail reach higher levels of power on their own, he can realistically cover some much-needed ground and raise himself to levels never before seen for his character.

Can Piccolo become as strong as Goku?

At this point in the Dragon Ball anime, Piccolo is several levels behind Goku, but a way does exist for him to become the strongest Z-Warrior again. ... The last time Piccolo was anywhere near Goku's level was in the Android Saga. As the fight with Perfect Cell approached, Goku grew much stronger and left Piccolo behind.

Is Piccolo as strong as a Super Saiyan?

Piccolo has been stronger than a Super Saiyan since he fused with Kami.