Calculate the Wet runway performance as normal (that is, with credit for reverse thrust for the RTO, and a 15 ft screen height for the GO case), and then do the same calculation for the Wet runway for Reverse Inop, and for a 35 ft screen height. NOW you have a direct comparison between the requirements for a normal takeoff and that with the allowed concessions. You might be surprised at the huge difference.
I am not in the habit of looking up certification rules (FAR Part 25) as a part of my normal performance calculations. However, I do occasionally check the operating rules (FAR Part 121)...
First, the 15% runway length buffer is mandated in Part 121 (121.195(d)) for landing only, not for takeoff as I had previously mentioned. However, a 15% buffer on the takeoff run IS part of the "all engines operating" criteria for takeoff in Part 25 (25.113(b)).
Second, Part 121 does NOT have a 15' screen height concession. That is ONLY part of the performance criteria for the 'V1 cut' case in FAR 25.113(b)(2). Note also that the takeoff run is the GREATER of the "all engines operating" case to 35' (including 15% buffer) or the 1-engine-out case to 15'. Therefore, there can be no "concession" to the 15' screen height if the all-engines-operating case is more restrictive.
Third, reverse thrust MAY be used on a wet runway IF its use is part of normal operating procedures. Whether or not that might make a difference in the calculations is a matter of individual circumstances.
After all that, operationally we all use our charts or performance computers per our airline procedures. I don't know if your airline allows you to pick and choose from among the different screen height and reverser scenarios, but mine does not. Again, I have NEVER seen a case in the 747 (manual charts in the -400, OPS computer in the Classic) where the wet runway performance was less restrictive than dry runway performance. If you can come up with specifics, I would be very interested in seeing them.