I don't think there's any way we're getting Joffrey's death in season 3. Honestly, if I were writing it I would even push the Red Wedding to the beginning of season 4, really take people off guard with major deaths in like episode 2. Jon's escape from the Wildlings, Arya's kidnapping by the Hound, Bolton revealing his betrayal and freeing Jaime, and Dany defeating Yunkai are enough for a good finale. I would end it there, then have season 4 open with Stannis and his leeches. Episode 1 of season 4 could then end with Balon's death to show it coming true, then RW in ep2 or 3, Joffrey's wedding around episode 5, Tyrion captivity and trial spread over the next few episodes, and then he kills Tywin in the finale.
That would set season 4 up as being about as eventful as they can make it, maybe give the showrunners their best shot at taking a best drama prize in awards season. And it helps with the budget. Season 3 is already packed with new actors they have to pay for. Pushing the need to cast Oberyn and any other Dornish they have him travel with (I think they may have him travel with Arianne to give an introduction to her and to better communicate her outrage at his death that leads to later events in Dorne) and any of the miscellaneous Freys to season 4, when they don't really have to pay for many new actors at all, except maybe some of the Greyjoys, frees up a lot of money.
Even if they keep the RW in season 3 though, rushing Joffrey's death into season 3 as well would really screw with the pacing. Season 4's King's Landing plot would be nothing but the trial, and having Tyrion locked away restricts the sort of new material you can make to stretch things. Plus, Joffrey's wedding really depends on coming after the Red Wedding to have its full impact. The wedding is a full-fledged victory party for Joffrey. His enemies are decimated, everyone's here for his giant party, he's getting the hot girl, and then disaster strikes in a totally unexpected way. There's a lot of satisfaction the viewer gets from seeing him smacked down from such a high point that you don't get if he doesn't go into the wedding aware of the fact that the Starks are destroyed and his war is won.
