Really, I don't know a better way to lower the center of gravity than by putting weight on the lowest part of the tank, underneath the hull! In the original post he only mentions increasing the weight, but weight evenly distributed under the hull actually lowers the CG and provides the weight as well as stiffens the hull, and doesn't overload the suspension in only one area which other types of weight might do, so you get four benefits in one.
While adding the plate does reduce the ground clearance by 3mm the Sherman has plenty of ground clearance. You run the risk of getting hung up on a rock no matter how much ground clearance you have, it just depends on the size of rock you try to drive over. Personally I try to avoid those big ones.
Steve