Actually you're both wrong; dividing by 1.15 doesn't decrease by 115% or 1.15%. Speed increasing nature gives a boost of *1.15, which is a 15% increase. Speed decreasing natures divide by 1.15, which I'm fairly certain isn't a 15% decrease (that would be multiplying by 0.85, which doesn't give the same thing).
EDIT: Posted about this in the SotG and apparently dividing by 1.15 is considered a 15% decrease by Deck, so that's what it should be.
Last edited by Flamestrike; May 4th, 2011 at 1:29:17 PM.