The speed and structure of themes play a role, but it's not a MAJOR role with regard to SEO, as the CORE of your website will be WordPress and it is a strong content management platform
Kallzu SEO theme is SEO Compliant. We have tested it and we can vouch for it
There are tons of WordPress themes out there which are good for SEO too. But we have not tested them and hence we cannot give you a clear comparison.
Your ranking mainly depends on a lot of other factors, and themes only play a very small part.
We recommend kallzu theme because all the kallzu plugins and features will be available to you if you use kallzu theme. Those features will not work in any other themes (eg: integrate email lead tracker from your site to kallzu 2.0).