If the you get a 3in DP and a 3in cat how does a Greedy TI or any 80mm that necks down to a fit the factory cat fit the new 3in cat?

 

I think you have to have a shop take out the neck down and weld on a 3" flang.

 

the TI necks down to 3 inches

 

Believe it or not, any good auto shop with a good rep with mufflers and etc. will have the mechanical skills to fabricate you exactly what you need. Not everything you buy are bolt-on, especially products came from different brands but work congruently, so expect the flexibility that fabrications can give you.

Just tell the shop exactly what you want and, with a little extra cash, you should be ok. As a hint, look for local shops that also are distributors of big brand name exhaust systems.