Here's something coming from someone who just went in the other direction.....
I had (still have, its all for sale in Classifieds) an E-PL2 with 14-42 with OEM flat port, Pana 7-14 with Zen Dome, and Leica 45 Macro with Zen flat port. To answer the OP's question, unless you must have fisheye, those 3 lenses/ports are all you will ever need underwater with a m4/3. The 12-50, which I have only played with in the store, could replace the 14-42 and Leica 45, although I seriously doubt its as sharp as the Leica. That lens is stupid sharp.
I loved the system and was about a millimeter away (waiting to see the price on the Oly housing compared to Nauticam) from going to an EM-5. Then, thanks to Stephen Frink, I came accross an incredible deal on a Seacam 5D2 housing that I just could not resist. I've been shooting Canon topside for longer than I care to admit, so no lenses to buy, etc.... Sorry to say, although m4/3 is very nice and, as Alex has said before, the EM5 can and will give 1.6 crop SLR bodies a run for their money; full-frame still rules.... but I digress.
On almost every dive (about 5-10 a week) I would have 2 or 3 decent shots that there is no way in hell I could have made with anything bigger than my E-PL2. There's the critter in the crevace, the low-angle upward shot under the rock, the slit between two railings on the wreck... list goes on Those are shots I will now absolutely not ever get with my 5D2, Seacam housing, and trashcan-lid sized SuperDome. I am hoping that what I gain is more than what I lost.
Moral of the story: You have an awesome system and you will be able to do things with it that us with physically larger setups cannot. Enjoy and welcome aboard!