@taichokage: Roshi's moonbusting feat is an outlier/a plot device. And there is also the daizenshuu which says KP only has the power of a small nuclear bomb (some people consider it non canon, so no problem).
Even if you don't consider it an outlier, there is nothing which suggests King piccolo can moonbust especially because the MAX kamehameha is a technique that multiplies the user's power level (while KP's demon wave doesn't).
The fact that he brags about busting a city clearly proves its a big deal for him.
-More evidence that KP is only a city buster -
Chapter: 161, P13.4
Yajirobe: “But man, what kind of a guy are you?...I can’t believe you beat someone who could blow a city to pieces…”
This statement by Yajirobe clearly implies that-
1. City busting was considered a big deal at that point in DB, and
2. Yajirobe (who is stronger than 21st BT Roshi), himself admits he can't beat a city buster.
Coming to Sensui's feats, the mountain he destroyed was scaled to be larger than Mt. Everest by two separate methods.
Not to mention, he's a whole league ahead of KP in speed. So as soon as the battle starts, all I see is Sensui punching King piccolo's head off before he can make a move.