The kinetic dump alone will be sufficient to do tremendous damage to the ship. Tremendous damage. There is a real reason why close-range anti-missile defense systems have gone from gun-based CIWS like the Phalanx and GoalKeeper to missile based systems like the rolling airframe missile. Why? Because against supersonic anti-ship missiles (eg the old Sunburn, and the more modern Yakhont and supersonic Klub) even if the gun hits the missile directly the fragments will be moving so fast that the ship will, basically, get blasted by a gigantic supersonic shotgun. There are some tests (check YouTube) of the Indian-Russian BrahMos supersonic cruise missile,, whereby it hits a target dead Center. The missile doesn’t have a live warhead, but the devastation is interesting. At certain speeds a warhead is not necessary - the kinetic energy alone is more than sufficient. That projectile hitting the side of the ship at hypersonic speed will not make a small hole - the internal spall damage alone will be ridiculously damaging. It is like a larger and FAR faster version on the depleted uranium round the M1A2 Abrams fires at an enemy tank. A slender metal dart once it is out of it’s sabot, and when it hits it goes in and the spall makes whatever unfortunate soul was inside that T-72 experience a truly unique death. The rail gun would be like a giant Abrams firing a shot at hypersonic speed. Whatever gets hit will not have a small hole - it will be finished (whether it is fires or, at the very least, a mission kill). It is the same way a slender uranium dart from the Abrams main gun hits a T-72 and turns it’s insides into slapped hot metal and sheared hot flesh - just far far far faster and a whole lot more kinetic energy dump.
Above a certain speed there is no kinetic dump. So that is a limiting factor.
See also Chainmails post #15
Add to above, the Navy has discontinued funding further research by the foremost expert the Navy has and he is the worlds foremost expert.