I work at an exhaust company and we do a lot of R&D on different type of cars.
Usually we remove the OEM cat, on recent cars it's placed very close to the engine to warmup up quickly and pass euro4/5 reg. We use a downpipe or a manifold without the cat and we place a sports cat downstream, under the car.
Just this usually brings a consistent gain, especially at high revs. On turbo cars the boost pressure increases a bit (if the ECU doesn't limit this) and the spool up time is reduced.
The downpipe is often 70 or even 80 mm in diameter, but it gets smaller at the cat, and sometimes even smaller at the centre/rear exhaust.
For the Golf GTI we use a 80 mm downpipe, a 70 mm cat and a 65mm muffler.
For NA cars it's even more complex, you have to consider primary and secondary pipes, lenght of pipes...the clio manifold, especially the 182 one, is VERY good.
I wasn't at the company at the time but they tried to make a better one but it was useless.
In the end we used a 200cpsi HJS sports cat and a 60mm exhaust (58, 65 and even 70 were tested).