"If he's really only using 50%" is Goku simply believing there's a chance that he's full of crap. He knew that a Kaio-Ken x20 wouldn't matter if Freeza was only using that much power, anyway.
Stopping the KMHMH was merely confirmation that Freeza was only using 50% of his power. There's no change in power, only effort.
In a nutshell:
Goku: "If Freeza is only at 50%, I'm going to get my ass beat to oblivion. Might as well try the Kaio-Ken x20.
*Shoots KMHMH*
Goku: "Well, looks like he wasn't telling a lie about using 50% of his power."
That's all the sequence really means, imo.