exhaust tip question

Here is the problem with the rectangular factory tips. Even if you like the style they are cramping yours..

They do not flow the equivalent of 3" round. They make the car louder. Do they flow the same as 2' or 2.5".. ? I don't know. But when I was buying my tips I did the math and thus avoided the factory restrictive tips since I used a TTI 3" exhaust.

I used silverline 3" to 3" double rolled, miter cut polished tips.