Headers: wrapped/coated vs. unwrapped/uncoated

Exhaust manifolds on turbocharged cars are used to channel potential energy to the turbocharger. Any energy that is lost to the manifold or ambient air is not getting to the turbo which means turbo performance will be reduced since the energy can not be used to spin the turbine wheel. This loss of energy means it will take the turbo longer to reach target boost.
Wrapping and coating manifolds is all geared to the same thing, to keep as much heat, or energy, in the exhaust system as possible. Exhaust wraps and coatings achieve this by acting as insulation between the hot manifold and the cool ambient air rushing around the manifold. By stabilizing the exhaust gas temperature reaching the turbo with the use of exhaust wrap or a coating, turbo performance will become more stable. Deciding whether to use exhaust wrap or a coating is ultimately up to the end user, but generally cast manifolds take better to coating than tubular manifolds, just as tubular manifolds and headers are easier to wrap than a cast manifolds since they have a more open design. One last note on wrapping or coating exhaust manifolds, be sure to check with the manufacturer of your manifold to see if wrapping or coating it will void the warranty as different manufacturers have different policies.