Just a final 'report' for the stumble/hesitation issue around 4500 rpm with O2 FI code appear/disappear (the latter happened when the problem present).
Since the bike already had the O2 FI code when I bought it last autumn, I didn't expected O2 to be the cause of the problem arised in late (warmer) spring. After a few hundred kilometers without problems I think the O2 eliminator is the solution to this, non-O2 models don't have this problem - I think.
Because I also suspected the TPS sensor ( the service manual is useless for fault finding by symptoms as it list almost every sensor for most of the similar issues ) I removed and checked it too which not only the manual, but I also not recommend. Setting the TPS (main) sensor idle voltage to 0,8 V instead of the "0,65 V or slightly more" in the manual (and/or the O2 eliminators) resulted in higher fuel consumption & sooty pipe end (instead of tan brownish) and even if I put back the KLEEN / PAIR valve, the exhaust rarely popped, so I'll try it soon with 0,53 V idle TPS adjustment if the theory is correct. Which is that higher TPS voltage could result in richer AFR, and lower = leaner. I've tried to find data on the web on this but this
is the only that worth to mention it. There's a chance to have idle and low rpm issues with this setting, but if I can get away with 0,12 V less voltage - as I got away with 0,25 V higher, then it will run just fine. If it will be okay , next ride's mileage will tell. (I'm only concerned about cruising mileage; my wrist movement & riding style efficient & steady enough I simply cannot accept 6,5 liters/100km for the same ride on which my buddy's gixxer L3 needs only 4,5-5 liters - we rarely did 6k rpm or above)
(Funny that I can adjust the TPS without disconnecting anything on the bike & then put it together within 45 minutes, I surely didn't wanted to learn that... :) )