Since he became a rubber man maybe his fourth gear is bounce back when he get's hit the power you hit him with can be absorbed and thrown back at the attacker what you guy's think.
The ability to store impact force is extremely useful as long you can use it later. Twisting his arm into a spring with extreme strength and a ratcheting system to store force for later would be so boss cause luffy's strength is already so much it would be like a kings punch.
He already pretty much has that, not energy, but he can now fire blunt attacks right back at you around the twice the power he said. Rayleigh helped him with it.