If I decide to get the DC headers, should I change the size piping to my exhaust? If so, what should I go with. I have a 99 tl

 

you don't have to, you won't get any gains from piping size.

 

no real need. If yo have the bling to you could though. I would put my money else where like Pullies and intake

 

for best result you can get a good set, free flowing of cat back piping, axle back won't work coz of the dick size of the stock mid-pipe

i have experienced noticeable power gain after i installed the comptech b-pipe and a type-S cat, and running with only a little back pressure

the only down side? probably spending 1000 dollar for the exhaust and it vibrates a little bit at idle due to lack of back pressure