Flying only through clouds without light to moderate chop is no reason to slow down. We normally don't reduce speed with every "bump" in the sky. There has to be at least sustained chop before reducing to "turbulence penetration speeds" which on the B747 is 290-310kts or M.82 to M.85, whichever is lower. Because of the large airplane inertia, at high altitude at heavy weights, it's important not to get too slow, better to stay at the upper end of the turbulence penetration speeds.
[Va speed is max maneuvering certification speed at which you could safely stall an airplane. During certification, this has to do with forces measured on the elevator. Above Va you may exceed structual limit before reaching a stall].
Hydroplaning speeds would be based on the 16 main wheels, rather than the 2 nose wheels, as directional control at high speed is an aerodynamic function of the rudder. The nose wheels, which have no brakes, have virtually zero steering effectiveness above 80kts on a wet pavement. The B747 nose wheels are identical in size and pressure to the main wheels.