yea its a piece of cake to install...40% is about the max, i've done almost a full tank of e85 before my flex kit but my check engine light came on because the computer can't handle the high octane. That's easy to remedy, just add more gas. I think the mileage can be about the same (it just depends on your vehicle) in my case I drive a huge suv and get about 2mpg less but the cost can be up to a $1 less per gallon for ethanol. Try experimenting in the meantime, when you are down 1/4 - 1/3 from full top off with e85 and see the improvement in performance! btw, I bought my conversion kit from jonnyenergy.com fwiw...
http://www.youtube.com/v/Q8Gawyybcwc&color1=0xb1b1b1&c