Hm, I have to admit I don´t remember that part that clearly. Wasn´t it so that Kami-Piccolo fought C17, got cocky and in the end would have lost? Or was it just that he intentionally let C17 suck dry one of his arms to deceive C17? Hm. I really could re-read the Cell-arc, has been a long time since I took a DB issue from my book shelve ;D

As for power levels, well, Kuririn is the only one to get anywhere as strong as he did. Maybe Tenshinhan could be counted, too. But imo you cannot explain that power level by continuous training, when only 2 out of billions of people have this kind of "talent".