Can you say "Mi parolas Esperanton, kaj kio!?"?
I think it's important when using Esperanto to remember that your target audience are unlikely to be native speakers of English, so translating an idiomatic English phrase such as "So what" into Esperanto word-for-word would probably leave them scratching their heads in puzzlement. I like Salivanto's suggestion of "Cxu gravas"? I suppose we really need to think what we mean by "So what?" when we say it in English, then translate the meaning into Eseranto