I believe this morning's hotfix took care of that.
As others have said, the SSD will only help with loading. There's not much that can be done about mobs/players being hidden, since that's a server function.
No, it's a server function. People only think it's client because they think the issue is graphical, but it's not. The server hides enemies and players because it cannot handle the load due to too many people being in one spot. So it cuts out things, which reduces the amount of things it needs to keep all the clients there informed of.
An example. 10 people in one spot, no mobs. The server has to tell each player, what the other 9 players are doing. 100 people in one spot, no mobs. The server has to tell each player, what the other 99 players are doing. Can you see how much more work this is?
Now, take that 100 people, or more from what I've seen and then throw in mobs.