Just to clarify on the O2 sitch.....
B1-S1 = Primary sensor. This one does all the fuel adjustments. Fitted in the downpipe upstream of Cat 1.
B1-S2 = Secondary sensor. This one measures catalyst efficiency. Fitted in the exhaust downstream of Cat 2.
B = Bank. B1 and B2 refer to the cylinder bank number, so the TFSI will only ever have B1. R32, V6 and V8s will have B1 and B2.
S = Sensor.
Never, ever fit a spacer to B1, S1.
If you really must use one, fit it to B1, S2.
All decats will trigger a B1, S2 catalyst efficiency fault code and a CEL.
Some Stage 1 remaps will also cause the same, but not always immediately.
Stage 2 and above remaps code out B1,S2 completely.
B1,S2's job is to make sure the cats are doing their job, but since it's downstream of BOTH cats, you don't know which one is buggered. A bit of a sh1t design really.
Not sure what's going on with that dyno plot, it just looks weird. Might have been an early run trying to dial in the N75. That is one humungous boost spike. Piston cracking cylinder pressures if that number was legit.