I might have to elaborate/edit later since I'm pressed for time FYI. Most important thing is that Oda clarifies "destructive power" for scenes like Luffy destroying a big chunk of Dressrosa with the residual kinetic energy from Doffy. The latest chapter gives the explanation, but the previous chapter confirmed how Luffy taps into his true potential with his life (or the life of someone he cares about) on the line. Thus, Luffy taps into and uses level 2 haki during his attack against Doffy. Thus, the "true Yonkou level" destructive power is from the combination of max Gear Fourth exertion plus level two haki. Big Mom's massive strength without haki is why she can destroy stuff at a similar extent as Luffy in Gear Third/Fourth. Big Mom using basic busoushoku means the entire mountain should go down with one strike (roughly). Big Mom using level 2 haki means the entire area gets destroyed (should be MUCH bigger than the area Luffy destroys in Dressrosa).