I think you are playing with fire if you really care about the warranty. I doubt a dealer will 'guarantee' that a preferred vendor will not break the warranty, and no vendor will agree to cover the warranty if their work doesn't pass. You'll have no options at that point except enjoy your new tune and to save up for any needed warranty work that won't be covered.
Personally, I don't get the need to do 'things' to the cars. I mean Porsche has 75 years of proven and successful racing/engineering/design experience and I just trust them to get it right. Plus, once you start down the rabbit hole of improve this, tweak that, it never ends. There are tons of vendors out there that would like to free you up from your money, and all of them combined don't add up to Porsche's expertise.
