You will gain a little.
I did not change the tranny in mine, just the final gear. I had to mill 15thou on the face and that was it. I have been saving fuel since then. Quite important these days as fuel prices in Quebec have skyrocketed to 1.45 a liter or about $6.00 a gallon.
Of course the gvnmt. is mighty happy. Taxes are on the total price not by liter.
Less IS more !