Not cool
I'm not saying that making certain textual changes or making your own little "improvements" isn't ok. That can be interesting and fun to hear. It's just that picking the right spots to do so is pretty important. Especially in Chopin, moreso than Mozart, or Schubert, Liszt...
And what you are describing here sounds pretty distasteful and ill-informed. A sf at the end of the Fantasie-Impromptu has nothing to do with the piece at all.
With Chopin, like with Beethoven, generally speaking, I don't think that one should change or add anything. It seems that their music, well, just doesn't take well to improvements, if you know what I mean.
And if you must do so, then you should have a better reason than "I am - - - , and I can do what I want".
Cziffra was very often a bully with what he played, he kind of bullied the music around to suit his own technique and abilities, as he said, simply because he was Cziffra.
He was great anyway, but very often misguided by lapses in taste, I believe.