Thanks for editing the excel sheet. I will take a closer look in a minute, but I was too enthusiastic to reply, so I did this first.
I think the formula's are correct (but I could be wrong).
If you increase the efficiency, the power from the wind increases. If you look at the graph you will see that it seems that the power of the generator decreases, but look at the values on the Y-axis: they have changed! In fact the power from the wind has increased, but the generator is still the same. So the power from the generator is lower with respect to the power from the wind. I hope this clarifies it!
For the value of 'power from generator':
The power of the generator is at it's maximum when the load is equal to the internal resistance. That means: if I have a generator with an open voltage 'voltage' and an internal resistance 'resistance'. And if I put a load with 'resistance' ohms, The current flowing through the load is equal to voltage / (2 * resistance).
Multiplying this with the voltage gives me the power.