Just want to say, you don't need to dyno tune it for every mod.
You don't have to retune it for every mod.
It will make more usable power.
O2 sensors can work.

My experience with running two strokes in closed loop and using O2 sensors has not be very rewarding.   The life span of the O2 sensos are very short when you use leaded racing fuel.  Even when using unleaded racing fuel, keeping the O2 sensor hot enough to keep the O2 sensor from fouling is just one of the many problems I encountered running a two stroke in closed loop mode.  Large single cylinder engines seem to give more O2 sensor problems because most riders do not run the engines at wide open throttle as much as the smaller engines.  Keeping the exhaust system real hot and the average exhaust temperature inside the pipe high is helpful in extending the life of the sensor.  Two stroke racing engines need a lot of oil in the fuel to meed the lubrication demands and this also causes premature failure of the sensors.  The LSU heated Wide Band sensors that Bosch makes seem to last the longest but the life span is still not acceptable by my standards. 

I started using EFI on two strokes in 1993.  The controllers we were using were supplied by EFI Technologies.  At the time they were supplying the engine management systems for the Cosworth and Ilmor Chevy Indy car engines.  We had the same systems and software as the Indy Cars were using but we were only using 2 cylinders of its full capability.  The system we were using could control up to 12 cylinders with 2 injectors per cylinder and each injector had its own fuel map.  These systems were sequential, allowing the tuner to start or stop the injector at any desired crankshaft position. 

The above system also had a lot of options as far as the different types of ignition systems that it could control.  We used the system to control our ignition and had a different spark map for each cylinder.

We were most successful running the Alpha-N type system which used RPM and throttle position as the primary variables for fuel and spark mapping.  The optimum air/ fuel curves for high performance two strokes are not nice and flat like the majority of the four strokes.  Four strokes are a natural candidate for using an O2 sensor to control the engine's fuel needs but I have never been happy with the quality of the fuel control that an O2 sensor provides on a high performance two stroke.