Well, that chapter in general tells us a lot about everyone's powers: Piccolo isn't mentioned to be fighting evenly, so he's below Vegeta and Trunks. He's also not stated to be presently in danger, so he's at least stronger than a weakened Goku. Where he's standing is between those powers, but fairly ambiguous beyond that. It is worth noting that earlier during the end of Goku vs. Cell the latter only mentions Vegeta or Trunks as Goku's possible replacements (and even then weaker then him), as far as the manga is concerned - so we know he's not perceived as that strong.