I just installed a Greddy TI catback and want to know if a DP will make it even more louder?

 

Probably...anything that lessens restriction in the exhaust flow generally makes the exhaust louder...

 

The cause and effect for loudness is sound pressure which in this case is directly related to exhaust pressure. The percieved sound level is a log function, it takes ten times the sound pressure to produce two times the percieved loudness. Ultimately, in theory it will be louder and perhaps measurably as well but you will not really be albe to notice much.. what you will likley perceive is a change in pitch.

 

only if you forget to bolt on the remainer of the exhast after the dp

Your exhaust will become louder only if you:
1) remove the catalytic
2) remove the muffler
3) remove the resonators with a mid-pipe section

 

Yes.