Back pressure in the exhaust is heat. The least amount of back pressure is optimal for the best power from a naturally aspirated engine. A lot of people believe that they lose low end torque when they relieve the exhaust system of a lot of back pressure. This is only true when the engine isn't tuned per the modification. Running an open exhaust manifold does releive pretty much all back pressure but not in an efficient way to make more power with it because the design of the manifold doesn't create proper scavenging (help pull the hot exhaust from the cylinder head. A longtube header is a must when running an open header with these engines. A tune specifically for an open header is also a must. An open header is only good for maximum power for wot. Higher back pressure usually equates to a slow and excessively hot exhaust flow. Catalytic converters create an obstruction for exhaust flow therefore increasing egt's to burn off any raw fuel particles and thus increasing back pressure.
With a turbocharged engine, catalytic converters are a turbocharger's enemy. The turbine blade should be the only obstruction in the exhaust path because that turbine wheel uses the energy in a high flowing exhaust stream to spin the compressor to produce more boost. If there are obstructions in the exhaust, that turbine will have less energy to work with essentially limiting its the potential to make more power. This is why most factory turbocharged car buyer/modifiers gut the cats or purchase a larger catless exhaust system usually increasing power and by at least 20-30hp (on a factory turbocharged engine).
I can go more in depth if you have questions.