Possible slight oil overfill and Fault Code P1923

[MapleSyrupJizz]

Hey!

First time posting here but a long time lurker.

About 4 months ago I got my dream car, Sprint blue b7 RS4 avant. Absolutely love it.

It came to that time again to top up the oil. I may have put a small bit too much in. The level was very slightly touching the max plastic bit. But thats not all.

I did this yesterday and all was well. I was driving home from the super market today and put the boot in, 3rd gear from about 2000rpm up to maybe 3500rpm and I notice a fairly heft puff of brown sooty smoke. Its never smoked before. Ever! Exhaust tips stay spotless even when the car itself is manky.
Im pretty observant when it comes to this kinda thing so if it had every smoked before I'd have noticed.

I got my mate to give it a few revs while it was stationary and I stood behind it and it smokes a small bit. Only a small bit to be fair but it only come from the left.

My questions are; Why only on the left? and why only now?

I understand these things smoke a bit, and thats fine. But why is it only happening now, coincidentally after I may have every so slightly put too much oil in it?

I got a diagnostic done on it and it threw up fault codes P1923 and P0491.
I've googled P1923 and its quite worrying. A sensor that can result in a busted Catalytic Converter. Can anyone shed some light on this?
http://p1923.enginetroublecode.com/audi-rs4

It had its carbon clean about 6k miles ago. Heres the thread of the previous owner getting it done. viewtopic.php?f=35&t=126446

Honestly, Anyone who can help I'd greatly appreciate it.

Thanks in advance
B