Here is a universal tip for great RP.
The more someone gives you in RP the more you give them. - That is if they give you some hits in a spar, make sure to keep it interesting and rp giving them an advantage too, so the the scene, thread, chat or whatever plays out with some tension and intensity. If they stumble so you can hear them coming, play out them having an advantage later etc.
If they seem overpowered, god mod, bunny whatever you want to call it. Don't give them very much at all, they can be drunk, mad, delusional in your character's eyes and the scene can STILL be fun. I have had a blast holding character with people all in caps, or breaking OOC - IC before, just depends if you can roll with it.
Imperials have hated aliens for as long as i can remember, if I RP an imperial human agent you can be that isn't going to change. Are there other ways of playing an imperial agent you bet, just not how I RP them. As for Sith its my experience Rawr Smash gets old very fast in rp boards or communities, and the writer or player burns out. You need a lot more too them to keep them interesting for the rp'er and others.