In my experience over the past 2.5 years with the FJO kit and the NTK L1 H1 sensor, the sensors seem to last a relatively long time with leaded fuel. I have about 40 hours of operation on leaded fuel on my sensor and it still works the same a new one (I recently temporarily swapped in a new one for comparison). I know numerous FJO customers who routinely run a good bit of leaded fuel and their sensors have lasted well.
And FWIW, after selling several hundred FJO kits in the past couple years I have only sold two or three replacement sensors because the owner thought the original sensor was crapped out.
Now, if you drive your car daily on leaded fuel, the replacement frequency (time wise) might be a bit rough. But for those who use the cars less frequently, the cost to keep wideband sensors alive on leaded fuel is insignificant relative to the overall cost of owning and maintaining a 700+ HP car.
Right now, for anyone who would feel more comfortable having a spare sensor, there are some NTK L1 H1 sensors available for $180 delivered in the continental US.
I am not sure if the L1 H1 sensor will work with the electronics you have, or not. Generally speaking, the electronics and the sensor need to be designed for one another, or else accuracy will suffer. However, since these are two UEGO sensors from NTK, who knows?
The NTK L1 H1 sensor was developed as an OEM sensor for Honda vehicles, and routinely last 40-60K miles (or perhaps more) on unleaded gasoline. It is a true UEGO sensor, so perhaps the UEGO you have will last that long or longer.