I think a Kaioken 3x would be enough. I generally have Imperfect Cell 2x Piccolo. Piccolo was rather weakened when firing the Gekiretsu Kodan, so I don't think it''s as strong as it'd be in case Piccolo was FP.
The way I used to view it, Piccolo would need 4x to have a lead on Cell. I don’t think that would be enough to get the job done unless there is no strain/unlimited use. 5x is what is needed for true destruction.