Canon 2X Ext - MKI vs MKII vs MKIII, how much improvement?

rickpoole wrote:

I borrowed a Canon 2X Extender (original version) to use when shooting the moon last night with my 100-400. Here are the cropped CR2s shot at 400 and 800 using ACR default settings (the 400 one was upsized to match the 800).

My questions are:

1. Would the 800 one would have been slightly better, better, or much better if I had used a MKII or MKIII version?

2. Are any of the extenders good enough to be better than just the 400 with 100% crop versus 400+2X with 50% crop.

The guys at the camera store said other than the corners being slighly sharper on the MKII and MKIII versions there isn't enough difference to warrant the additional cost. The reviews seem to say the MKII is slightly better than the MKI and the MKIII is very slightly better than the MKII (optically speaking).

Has anyone done a similar comparison between the MKI, MKII, and MKIII versions of the 2X Extender?

100-400

100-400 + 2X MKI