Hardness takes a very far back seat to fit in the barrel. Soft bullets do not lead the barrel in 9mm. Undersized bullets that get gas cut do. As far a polygonal rifling goes. All of my Black Powder pistols have polygonal rifling. They all shoot the softest lead you can get. I have a 9mm Glock 26 with the factory barrel. I have not shot lead in it. I use plated bullets due to the fact that I have 4 thousand of them left. When I get to one thousand of them I will slug the barrel, and get a mold for 9mm. (Note I have 3 other 9mm pistols.) I will try a few and see if it leads after a couple of shots. If it does then the lead bullets will be shot in the other 3 9mm guns I have. The Glock will then get plated bullets.
It is a personal choice for each. I have met, and seen several people that shoot nothing but lead out of factory Glock pistols. Two them swear they have never fired a jacketed round out of theirs.
Note as far as a the factory waranty goes. If you shoot ANY reloads out of it the waranty has been VOIDED.
Oh and as far as dead soft lead leading at 9mm velocity that would depend on the powder. I push darn near pure lead out of a .45-70 Govt. At over 1300 FPS with no gas checks. No lead in that barrel at all. Fit trumps hardness greatly. In fact at speeds under 1400 FPS you can push pure lead that fits with great accuracy. You just have to find a good powder to work with.
In the end it is your gun, and your decision.
No matter how many times you do it and nothing happens it only takes something going wrong one time to kill you.