Exhaust question

A friend has an '03 Tundra with a 4.7 engine. He took the muffler (http://www.toyotanation.com/forum/showthread.php?t=301106#) off and ran dual pipes after the resonator. Sounds good and seems to run the same but was wondering if there are any negatives to doing this. Thinking about it for my '06.

Rob