Every tune is different, and most tuners won't put the timing into an e85 tune, because the power is usually shooting up by that stage and they don't want to damage your engine. Why don't you speak to the guy doing the tune, as he would have a better idea of how hard he is willing to lean on it...
From my experience you are looking at around a 15-20% gain on ethanol, possibly more if the tuner is willing to lean on the timing. Doing so will cause exponentially high cylinder pressure, which can easily lift heads and crack blocks. If you want more power I would be simply leaning on the turbo harder before running more timing.
There is very little difference in response from my testing, but the midrange will certainly improve. All in all a great way of making extra power with more protection, by simply filling at a different pump, why wouldn't you run it...