From what I understand. The percentage of meth to fuel would have to be in the tank, and rather large to take up the ratio of 95:99, meth is over 100 is it 114 I can't remember off the top of my head.
I don't think a water/meth injection system would be the best way to increase the octane, and purely only used as that.
From comparing dyno runs using different octane fuel, for example 95 to 99, the power increase on high performance engines is usually noticable, from the dyno used on the first post on this thread the power increase with water/meth injection wasn't anywhere near as much as the improvement from a different fuel, so I wouldn't of thought the extra octane in the meth would be able to account for the loss of octane with the lower rating fuel.
Plus the fact, if it was a 50/50 mix, the amount of meth required to increase the octane rating to 99-101 would also include enough water to reduce power I would of thought?