Tuning by the oem O2 sensor is just slightly better the just guessing.
Using a GM O2 I had 12.5 and 11.8:1 at 830.
Hardly accurate.
I would try some more fuel.
If there is no lose in performance the add some more.
You'll get much better egine life being border line too rich then too lean, given the same EGT, and yes you'll have to work with the timing some to get the best results.
I think that is a pretty decent reading. My car happens to like that exact range for peak HP, but you will have to find out for yourself what your car likes.
I ran my car this past weekend around 1525-1550* just to be on the safe side while I was bracket racing. I could tell the car was way down on power but I wasn't worried about it since I wasn't trying to push the car too hard.
Also, if you are using EGT I would ignore the oem O2 sensor all together. If you want to correlate the two readings you will have to do it on a brand new O2, but they won't match for long. You will be surpised how quickly C16 skews the readings on your O2.