The tubular exhaust manifold you have is one of the best ever made and are practically headers. You will be lucky to get 2HP from a set of shorty headers. Some people have even seen a HP loss from them. Stick with the dual exhaust and don't replace the manifolds, UNLESS you are going to get a power adder, turbo or supercharger.
__________________ 2008 Mustang GT Premium 5 speed, 3.55 gears, black-on-black, IUP, 18in polished aluminum wheels, FRPP GTA Axle Backs, JLT II CAI w/SCT X-Cal 2. 296HP & 311TQ
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