This really belongs in engine management forum.
The stock tune can take a fair amount of E85 before running out of ECU adjustment range. However, a basic stage 2 will run out of injector and / or fuel pump before it runs out of ECU adjustment range. so will lean out even if the ECU is given time to adjust fuel trims for the new mix in the tank.
Some of it also depends on how the car is tuned:
There are multiple load ranges that use different fueling trims. These trims also get applied to the open loop fueling map. (For example: If the ECU is adding 5% in closed loop fueling, it will add 5% to the open loop map at that load range too.)
Some tuners will adjust these ranges so that the highest load range is so high, you never get there in closed loop, so there are never any adjustments to the parts of the open loop map that people are in most. If this is the case, you can cause real issues with running lean on just a small% mix of E85.
However, if a tuner leaves these load ranges reasonable, then you can add 20% or more E85, and the ECU will automatically adjust for that (if there is enough fuel pump and injector to do so.)
So, IF you have larger injectors + fuel pump, you need to confirm with your tuner before you go adding E85 to your current tune. If you don't have larger injectors + pump, you definitely don't want to go adding E85, as a Stage 2 is usually very close to maxing out the fuel system.