This gave me a great chance to get the RS6 on the dyno again to see if the APR exhaust and high flow cats made a difference.
Short answer. No.
Ambient air temp according to the sheet was around 23 degrees. Final run produced 250.5kW with 531.5NM of torque. Chatting to the guys there was around 2 to 3kW difference between the 3 runs.
Interestingly though, this chart shows Tractive Effort (N). From what I could work out from various sites you can convert this back into NM by taking the tractive effort then multiplying that by the rotational diameter of the tyres then dividing it by the gear and differential ratios. I ended up with 7600*.330/4.718 which works roughly out to the 531NM mark.
The stock car ran 257kW and 535nm on another Dyno Dynamics dyno (usual machine, fuel, air temp, etc. differences spiel). The modified exhaust produced the same power. Looks like the stock exhaust is going back on at some point. I personally would not pay the couple of grand that APR are asking for little to no improvement in overall power. As someone has pointed out though, this could be because the exhaust was designed around the APR chip. The better flowing exhaust may be beneficial to the higher boost and power that the chip produces.