I tried the strategy with Typhon and Mengsk from this thread:

http://boardgamegeek.com/thread/384152

We played a five player game I set myself up with Typhon and Erebus station putting my starting base onto typhon so that I started with the extra crystal. None of the other players even had a chance because of the defense you can set up on Erebus. Mengsks stage two leadership card means you have five incredibly well defended points. I effectively won on turn three with this because the other players realized it was impossible to keep me from winning the next turn.

After this I might make a house rule that keeps Mengsk from picking Typhon with his leadership card. Of course you don't have to worry about his if you don't have the Typhon planet.

Well I do not think that this would be some kind of ultimate weapon actually quite a contrary. Well in our games such a good position for mengsk would mean a certain suicide because other players would join together to destroy him. It would simply be too dangerous to keep him in the game. And combine assault of three players would smash him no matter how massive defenses on erebus are

Our games simply have a certain autoregulation mechanism... when one player grows too strong others join against him. So the strategy is simple. Do not be the strongest player in the second stage so you may have a chance to steal victoty in the third stage

Trust me the logistics for multiple players attacking erebus are very difficult, especially if you can connect it to a planet nobody starts on that has only a few sectors.

It is fairly broken even with the "kill the leader" balancing your talking about.