Well the s8 exhaust system stock is actually decent, not perfect sizing but it works and has a crossover. On the s6 you see better gains because there are worse bends, no cross over, and I believe slightly smaller diameter.
I mean if you removed the stock cats you should still be seeing some gains unless something else is going on, perhaps they are higher in the rev range though. Until you get tuned it probably won’t be huge. You should feel a difference though as you did remove some restrictions. A well designed exhaust does make a difference, but this is defitnely better than something like the milltek where I believe people were losing or seeing no more power over stock.
I really think unless you plan on doing full blown headers at some point and/or a blower than 2.75 is more than great! I defirnely noticed a gain and better throttle response on my last s6 with my simple exhaust setup and stock cats.