We would choose a wet V1 where braking co-efficient is reduced because of the wet surface. Typically this speed is 8-10 kts lower than V1 dry. This is great for the stop case but in the go case with an engine out it means that the distance to Vr has been increased and TODR increases. If we still had to achieve 35ft within TODA then the mass would have to be reduced. As the event is unlikely the screen height is reduced to 15ft to avoid this penalty.
CS25 defines when we can use reverse thrust for the stop case. We can't use it on a dry runway but it can be considered on a wet runway provided symmetrical reverse thrust is available. On a twin this is not the case.