Zeiss 50mm 1.4 ZE vs older Contax mount Zeiss

Hi,

I've been using an older Contax mount Zeiss 50mm 1.4 on my 5dMkII via an adapter. I just bought a ZE version of the identical model to take advantage of the native EOS mount. I noticed a decrease in wide open performance, specifically the center sharpness.

Did I get a bad copy? Is my old lens an exception? Is Zeiss more marketing now that performance?

Note I did this test on a tripod with a remote release, and locked down the ISO to 100. I focused using the Live View magnified x10. No matter how hard I tried, I couldn't get the new Zeiss to match the old one. The new lens is simply softer from 1.4 to 5.6.