OK, AVGAS is a DRY fuel, thus you should never go past a 10% mix as this will cause problems with fuel lines, fuel pump, injectors and so forth. Too much CAN actually cause more damage than the "perceived" benefit.
Also as it is a "dry" fuel it does not actually "mix" with the other fuel in the tank (similar to oil in water), however, that being said as both are fuels in the broader sense they do interact.
There is a slight performance gain, similar to that of using toulene or ethanol, it reduces the chances of knock and detonation and allows greater timing advance.
I have used this in many cars, purely at the track and NOT for everyday driving as the constant use is not good for the fuel system as mentioned previously.
Finally, with regard to the O2 sensor, too much avgas will not only see the death of this but also your cat. However, these effects are SIGNIFICANTLY reduced since avgas is no longer lead based. Thus a 5-10% mix, not on a constant basis should not have an effect.
I usually run 5% and have never had an O2 sensor fail, nor any other fuel system problems, I have seen 50/50 mixes do some nasty things to fuel pumps though...