That's not how that works. The saw will make the same power as it would on gasoline, you'll just have to tune super rich and burn 1/3rd more fuel on E85 to do so.According to this, ethanol only contains about 2/3 of the energy (67%) of gasoline per unit volume (say gallons), so E85 is only going to have about 72% of the energy per gallon compared to straight gasoline.
Start adding altitude losses (3% per 1000 ft elevation) and tuning losses and throw a dull chain into the mix and it ain't gonna be long before you gotta start burning some of that firewood to power your saw...