I can understand his reasons, but nah, Piccolo was adamant at Goku not finishing his fight against fat Boo since he kinda thought he could do it and later on Goku implicitly admits that he could have done so. Against Kid Boo, he fought him teeth to teeth and he could have won only via a concentrated blast, that's the impression Vegeta's line gives. If Grey Boo were > Kid Boo, that'd make Fat Boo way stronger than Kid Boo, thus Goku would either get annihilated by fat Boo or he was playing around vs kid, both of which doesn't make a lick of sense. I get that Good Boo managed more against Kid than against Grey, but I think Kid Boo wasn't at the best of his abilities initially due to being kinda annoyed at being attacked by one of his kind. Later on he dominated the fight and fend him off with his feet alone. There's also the argument that Good Boo was a little tired after expelling Grey Boo (visible in the pages) so he could have had his performance somewhat affected by that.
I think one could form the argument that Fat Boo only showed his full power when angry after Satan being shot and THEN he surpassed Goku and Kid Boo, also producing a being (Grey Boo) stronger than Kid Boo, but nah, I don't buy that either.