You already answered your own question. Any more time spent here is wasted.
"I only like the SZ tip design" - well.. there you have it then. What difference does the db's make? You already have 3"->2.5" test pipes so 2.5" SZ cat-back it is. And yes, its going to be loud. Should have stuck with 2.5" components and cats/resonators if you were concerned about sound.