@SingingThroughTheStorm:
When applying certain rules of reality, it does make it seem that Shigan is possible.
Remember, the CP9 members went through intense training to obtain certain abilities.
So with that in mind, if a human practiced and train for a year on this one technique, it should be plenty possible.
Tekkai just doesn't work though. There is no way (to my knowledge) you can make any body part like metal. You can't make your face more resistant to a punch.
Yeah uhm, shigan is not possible.
The bones in your finger would simply break or dislocate. There's no use training against that you`d never grow your finger strong enough to create bullet like wounds in their body.
As for the other technique's:
Kami-e: Noooo, in reality is hardly even works on paper, since paper doesnt "dodge" you trying to punch it. A human body is just way to heavy.
Geppou: Nope, you'd need feet the size of wings to push enough air down.
Rankyaku: No, same as Geppou really.
Soru: No no, no person in the world can move faster than the human eye can see, and the kicking the ground ten times thing is just silly in reality.
Tekkai: Not in the way it is displayed in One Piece. You can make you body pretty strong by spanning your muscles. But if the force is strong enough however your bones will either break by recoil. If for some reason your bones would not break your internal organs would get damaged. This happens to a lot of adults when in a airplane crash. They brace themselve upon impact and indeed manage to stay put, but the impact is so devestating it damages internal organs or in fact can even rip them out of the owners body.