Join Date: Sep 2016
Location: Eagle County, CO
I have not had a noticable soot buildup like you describe, either with the stock or LeoVince exhaust.
I cant imagine the stock vs aftermarket exhaust is the issue save for a leak at the header ports, but I would be pursuing the dealer for a replacement sensor, as they can be expensive and finicky parts. The service manual has a procedure for diagnosing the signal from the O2 sensor but it requires a very sensitive multi-meter, and will be easier to have the dealer do that and replace it.
2016 Versys 650LT - Madstad, T-Bob, LeoVince, SeatConcepts, HyperPro CSC Damper, Shoodabeen Flashed
2003 KLR 685 - +1mm IN & EX SS valves P&P, Innovate Wideband O2 Sensor, Progressive 465 w/ RAP, ThermoBob