I don't see why the wagner wold become superfluous. The niche of the wagner would be : amphibious tank. It will have more torpedo dps. It enhances its role as amphibious tank instead of making it good for everything, and on top of that amphibious.
With 4 speed, it will be aslo a good raider unit, coupled with mobile stealth deceiver that also has 4 speed, it can be nice also on land maps where speed matters (like open palms).
For now, the rhino is precisely superfluous, because the supposedly "speciliazed tank" is better than the supposedly default tank in all situations.