GaryLager,
a couple of comments here have confused me slightly
I don't want to touch on V1 as a stop/go/decision speed, it's another debate that's been covered in great depth a number of times on this forum.
I believe Woof etc was originally asking 2 questions:
a) Why would you have a different V1 for a wet runway?
b) Why would you have a reduced screen ht (15' as opposed to 35')
I believe I answered both of those questions:
a) Because of degraded braking.
b) Beacause of a greater TORR.
S15