I don’t think Rath needs and ult simply because it is one and only redeeming quality. I think his ult is super strong on purpose. It is part of his design.
I’ve been on both the receiving and giving end of pure Rath ownage, and then also on the giving and receiving end when the opposing team knows how to deal with it.
If his Ult wasn’t so powerful, there would be no point in picking him. His Life Steal isn’t good enough to sustain him, which is how characters similar to him in other games stay viable. If you could get gear that added decent lifesteal to his already small amount, then I would agree that lowering the numbers on his ulti would be viable. But as the game is now, I think he is a character that can be left as is for now.