It doesnt make sense though to say "if somebody keeps training, he´ll get stronger". Then we´d have thousands of super strong humans, because they are lots of people on earth that train their entire life, yet wouldn´t ever reach a 1000th of Kuririn´s power level. There´s clearly some barrier that normal training cannot pass. So I question what Piccolo could have done to break such limits.
Regarding Piccolo and Cell, it depends. Is base Cell stronger or weaker than C17 at that time? Because Kami-Piccolo got beaten by C17. I assume even base Cell was stronger than C17.