I had a custom twin exit exhaust made and performance seems good but they only used 50mm piping for the centre section splitting into 2x45mm pipes at the rear. Now the area of 2x45mm pipes is the same as 64mm piping so that should be okay

Checked my header and it has a 57m collector but it reduces to 53mm just before the flexi pipe and then 50mm after the collar. .

I am going to get the centre section replaced from the 57mm collector to where it splits into 2 pipes. Bearing in mind that the CAT has been removed which helps the flow tremendously, what size piping should I go for. 57mm right through or possibly 60mm. I think that 64mm might be a bit too much as the removal of the CAT has freed the flow up a lot already.

If you still work in inches over there, here are the equivalent sizes.

50mm = 2"
57mm = 2.25"
60mm = 2.36"
64mm = 2.5"