It's gonna be loud with the muffler ending under the vehicle anyways. A large opening tip will amplify it some but again, why? It's retarded to put an exhaust tip under a vehicle. What are you trying to accomplish?
Tips actually act as echo chambers and make the tone deeper. A lot of people take tips and put on end of their exhaust when dumper. Try it without and try it with and you will see the difference in tone it makes
Only tips that are oval or rectangular in shape and larger than the exhaust piping will even have a minimal effect. If you're using a regular round tip it's not gonna do anything different than a turndown would.
i have a flowmaster 40 series single in and duel outlets i have to 3.5"-18" echo tips welded to my muffler as dumps that was just because i had the tips and liked the way they looked inside the wheel well... they did make a quite a bit of difference than the 2" turndowns i had but i dont know if i would have paid to have them instead of regular turndowns. but i do really like mine