most of todays exhausts are overly restrictive as besides emissions noise restrictions are necessary, but an overly sized system can reduce torque + possibly mpg's while moving the power band to higher rpm ranges. i have the smaller titan aka frontier 2011 with the 4.0 V-6 + i installed the slightly bigger single doug thorley system, i like the sound + might have better power + mpg's, but changing to lower friction top quality Redline fluids all around prolly helped as well. start with the cat or cats they restrict the most + act as mufflers as well, then theres the muffler-mufflers + maybe some resonators!! duals cost more + add weight + you may need another sensor depending on the setup as i not titan savy, so a moderately larger less restrictive system would prolly be better. sound is another item as systems get bigger they get louder + droning can be an issue that depends on what you like, i would check the titan forum for opinions or talk to local owners. i wanted 304 stainless but its only available for titans, Borla it top of line 304 stainless + lasts forever, lesser cheaper 400 series stainless surface corrodes but lasts, + lastly cheap muffler shop mild steel rusts quickly but in warmer drier states its good for a while, your $$$ your choice