Yes, that stands to reason I suppose. Ethanol, I believe, in general has a higher octane rating than standard petroleum based fuel, so the more of it you use, by that reasoning, the higher octane the blend will achieve. If an engine is built to properly take advantage of that (higher compression, rather than just spark advancement for example), it would naturally be more efficient than one that was not.
I see macadam has basically covered this point in his post as well. His points about corrosiveness are good too. I admit my knowledge is limited here. What I would say, then, is that, building an engine to run on higher concentrations of ethanol may require the use of certain materials which aren't used in general for engines that just run on E15 or lower. It would probably not be a good idea to go testing out E85 blindly or mixing in E85 with your E5-15 just to see what happens.
Originally Posted by mhabs
As far as E85 efficiency, I thought E85 can be well harnessed in turbocharged engines with reasonably high compression ratios, if the engine is specifically built/tuned for E85. Here's something that was tested on a vehicle from now defunct Saab, where more power was acheived with no penalty in efficiency: