Sean the current car was BW K04-001 (old stock type), 2.5 inch dump+ cat + back section with standard unported mani. When I say unported it seems to have been gasket matched at the flange but not ported where it counts eg runner cyl 3 and into the collector. The gasket matching of the runner openings I dont think would have achieved much because the exhaust ports in the head were untouched so oversizing the runner openings would only have created a step.
So first I went to my old cars 3 inch dump, 3 inch cat, step down to 2.75inch post cat and then UGP 2.5 inch back section. The boost (indicator of less back pressure) I was seeing went up from 21psi to nearly 23psi.
I then fitted up my old cars properly ported exhaust mani. It wasnt gasket matched but had porting where its needed. The boost basically was going over 25psi and sailing north. It'd have hit 30 psi for sure the speed at which the needle was going up. Didnt see where it got to because the foot came off the accelerator quick smart. I backed off the wastegate pre load from 3mm to a whisker below 2mm and there was no change. Was basically behaving like a turbo with a wastegate hose leak.
So while the bigger bore exhaust no doubt reduced back pressure, the ported mani had the grand daddy effect on back pressure (albeit in conjunction with the bigger dump) reduction and basically made the APR K04 tune unable to control boost. I've had to use a parrallel MBC to clip the boost peak down to sub 20psi and the car is way under fuelled so the extra air flow is undeniable. Basically you'll be able to either get the same power with less boost OR more power at the same boost OR if your turbo was underdone as it was, be able to turn up the boost without running it right off the comp map.