If your horse isn't softening to the bit in a single jointed snaffle, but you saw an improvement in a waterford, then it's quite likely that he's getting too much nutcracker action from the original bit.
If the kimberwicke you try out also has a single jointed mouthpiece, I suspect you'll see the same problem.
Why not try the waterford a few more times? I personally like it better than kimberwickes because it doesn't use leverage. After a few rides in it you should definitely know if it's a good bit for him or not, and if it's not then you can try another (kimberwicke, pelham, etc)