4/3 to m4/3...what to do?

Get yourself an m43 with a kit lens for those times when you absolutely need decently fast AF.

Keep your existing lenses.

Keep also in mind that some m43 cameras seem to have a problem achieving accurate focus with the 50-200mm SWD, with or without the 2x converter. My EM-5 steadily misfocuses with any 4/3 lens over 200mm (I haven't tried it with the 70-300mm though). That's the 50-200mm SWD and the Sigma 135-400mm. It locks, but locks anywhere but when it should lock. It's always front focus or back focus.

No problem with shorter focal lengths.