below is a bit of info I found regarding exhausts, whilst I don't plan to change my exhaust straight away, allowing for a larger bore would be good - say 2.5", and even if I never do, a larger pipe doing a 180degree bend and acting as a buffer for lower backpressure wouldn't hurt would it?
"Because you want the least backpressure in a turbo car's exhaust, the ideal exhaust system would produce the least backpressure immediately after the turbo. Due to routing, emissions equipment, pipe diameter, exhaust gas temperatures/pressures, the perfect diameter changes from car to car, setup to setup. It's very difficult to know this without extensive testing, so as a rough rule of thumb, a consistent or increasing diameter exhaust as you head downstream towards the tailpipe is best in most cases. Mandrel bent exhausts are also always better. A mandrel bend is when piping is bent with a mandrel, or insert, to keep the inner diameter consistent at the bend. Crush bends reduce the diameter at the bend and reduce smooth exhaust flow. Most factory exhausts are non-mandrel bent crush style bends, so switching to a mandrel bent exhaust will increase power and efficiency of the turbos and engine with no other modifications. You also want to avoid very restrictive mufflers, sharp changes in piping diameter, and sharp bends. As a rough rule of thumb, each 90o bend in the piping has about the same resistance to airflow as 25 feet of straight piping!"