I'm a bit sceptical about any claim of 5hp gains from just spark plugs. That just sounds "fishy" They may be a good plug but additional hp from them.....ehh... I asked the same question a month of so ago and the answer I was given is autolite plugs.
NOT according to 1000's of people, myself being one of them, Platinum runs too hot for a cylinder head that only has 4 threads..I have heard pretty bad reviews from people running plugs of that nature saying that they would get hard starting, rough idling and missing. In my opinion, Auto Lite Platinums are the best plug by far too run in your mustang.