Canada Canon 2x ver. III vs. II on 300mm f2.8 IS I

I'd appreciate hearing anyone's experience who has used/compared this two TC's with the 300 2.8 IS I lens. I've used a lot of TC's (both 2x and 1.4x) over the past 20 years, but the this Canon 2x II just seems quite poor when used with this lens. Any comparative comments on the new version III would be much appreciated before I invest in one. I've done a pretty wide web search and have found that generally people think the III is better, but I value the input of fellow nature photographers most. Thanks.

Scott, here is a link to a test of the Canon 2x II and the 2x III extender. It appeared in a discussion one year ago. It is not specifically concerned with your lens, but it may be helpful nonetheless.
http://www.deepgreenphotography.com/...ty-comparison/