Did you read what I wrote? I said ''He isn't much faster than Kakashi'' not ''Kakashi is faster''.
It doesn't matter what did the damage regardless, the wires or the tags.
If Hidan can be cut by simple wires and explosive force, Gai's punches (which have tons -literally, tons- of force behind them) can tear Hidan apart.
With enough speed and power, a similar effect to cutting can be produced.
If I have a metal cube, I can cut that cube with a welders cutting tool.
With enough speed and power behind my punch, I could also shatter that cube with one punch.
That doesn't matter at all because of two reasons:
1.) It didn't rip Kisame's body at all.
2.) Gai wouldn't know that he needs to rip his body apart, seeing as how he has no intel on Hidan.