My car is not flex fuel but I do have dedicated e50 and e85 tunes. My understanding is that e85 is pushing the limits of volume that the stock fuel system can supply. e50 is still around 102 octane, so it has plenty of knock resistance, but does not require as rich a mixture. Hence the volume of fuel required is more comfortably within the capability of that which the fuel pumps and injectors can supply. Therefore the tune can be more aggressive. Testing on various other engines has shown that the majority of benefit is achieved around e50-60 and beyond that you reach the point of diminishing returns except in all out race engines.