After cams install and remount of all the exhaust parts, the car developed the fault in the topic.

The AFR's seem to be alright at WOT, but they're way off during idle/cruise.

So narrowband sensor in the o2 housing or is there any other possibility?
Car in question is a 2003 VIII.

P.S. Is it safe to boost around like this ?

 

Have you checked your fuel trims? Idling at 12 is pretty rich and if its pulling +/- 25 then that would be why you are getting the code. You can check your o2 feedback with evoscan to see what it is doing. Cams will change your fuel mixture and it could be throwing off your closed loop tune.