This is the old version of Piccolo that Goku was able to win against. Why does Mutaito always get nerfed? Seems like him, young Roshi and young Tsuru were beasts.
This is a very good match up. Goku ( vs Piccolo ) is stronger than Mutaito as well as Piccolo ( Initial ). I think Goku being stronger than Mutaito for the same amount he was against King Piccolo ( Initial ) looks about right.
Goku ( vs King Piccolo > King Piccolo ( Initial ) ~ Mutaito ~ Karin
Chapter: 143, P6.3
Context: after Piccolo and Goku fight a bit, before Piccolo gets serious
Piccolo: “To think that there’d be someone in this world who could knock my back to the ground…I see…At that level, it’s no wonder my underlings couldn’t handle you…”
Chapter: 144, P7.1
Context: after seeming to kill Goku
Piccolo: “Naturally, this level is about as far as a human could go.”
I agree, it is possible Goku got a small zenkai from his accumulated damage from Tien and Tambourine. It's not definite, but it's possible.
Tien seemed to consider Jackie a real threat and was generally very impressed with his power. It seems implied that Tien would have to work really hard to beat Jackie at full power.