Read the E85 FAQ here: http://forums.nasioc.com/forums/showthread.php?t=803341
The guy who wrote that ran E85 blend on the stock ECU for years before converting to E85 and re-tuning. There is significant benefit to be had from mixing E85 on a stock ROM due to leaning out the stock richness in the open loop fuel map.
If you're tuned for 91, however, it's obviously not a good idea, as that has already tuned out the stock richness. It depends how they did the tuning how bad it would be. If they moved the ariflow ranges of the trims so OL fuel map isn't affected by the trims it would be worse than if they left the trim ranges alone.