That's a tough call. And ultimately you are the only one qualified to make that decision. Penchum wrote a paragraph in his review (in the OP) that pretty well sums up his take on the MKIII-IV-IVse lineup. It's, of course, just an opinion of one guy, so you can put as much stock in it as you feel comfortable.
He seems to be a fan of Little Tube products, and there is nothing wrong with that (I am too), but you must consider any gear purchase as objectively as possible imo.
Personally: I thought the $200 was worth it. I don't want to buy a product and think, "It's nice, but would I be happier with the higher-end model". Of course the answer to that is almost always yes for me. Then it just comes down to, can I afford the additional cost. If I can, I do it. Is it twice the amp? No, but with almost anything audio, there are diminishing returns the higher in price you go.
EDIT: SilverShadow. I have a question for you. What gain setting do you use when you run your K701's through your MKIV? And does your amp run hot with this pairing? I've got mine on the gain setting of 10. I usually don't have to go past 12 o'clock on the volume knob but the amp gets very warm.
If I were to buy the MKIII, I would probably end up buying the MKIV SE later anyhow, and therefore spending the money twice. The I+ is good enough to satisfy me in the meantime. (I have ruled out the MKV and MKVI. The V is solid state, and my experience with the I+ has taught me that I prefer tubes. The VI is balanced, and that would require an expensive recable for my phones.)