Cooler EGT gives you room for more power. Thus water/meth can do more than just add ~10-15hp right off the bat.
Take TDIRacing's dyno, for example. He gained 13hp in a very wide RPM band by just adding 50/50 water/meth:
http://forums.tdiclub.com/showthread.php?t=194165
He mentions that his EGT used to peg 1600F at the end of a 4th gear pull, but since adding water/meth he could just touch 1400F. It might be feasible to see another 5-7hp by adding more fuel. Or you can raise your RPM limit while the EGT is slower to climb, which might give you an advantage on the track.
The power gain could total 15-25hp with water/meth and a retune. It doesn't matter if you consider water/meth a direct or indirect power adder, it is still a power adder. If you only want cooler EGTs otherwise, why not just reduce the aggressiveness of your tune a bit and you don't need to bother with water/meth?