Chapter: 482 (DBZ 288), P7.2-3
Context: after Super Saiyan Gotenks forms for the first time
Piccolo: “…Yeah…Your ki really is absolutely incredible, but how about your movement?...Show me a little.”
Gotenks: “Is that alright? If I show you here, the house might break. I’ll do it on the ground.”
Piccolo is sure on his ki level being enough to defeat Boo. He figures Goku could do it as well, but he still had to ask.. With Gotenks he seems confident, however he wants to solidify his thoughts by viewing his speed. Given how arrogant of a tool Gotenks is, and how inexperienced the kids are in combat, I think having a sizeable gap like 1.2x SSJ3 Goku makes sense.