@tilraen I always use custom themes (built from scratch) on important sites. On support sites I don't care how optimized they are because I don't need them to rank. That being said, I know of no theme to be 100% SEO optimized by my standards. There are many who say are optimized and stuff but most themes advertised as SEO optimized are crap in terms of SEO.
One other thing is Wordpress is fro blogs and a site is not a blog. A blog is a limited from of a site but if you're serious about your business and conversion rates then I hope you people aren't using a WP theme/setup that looks like a blog. It kinda works for MFA sites that you don't care much about but not for a serious site you want to be truly optimized for conversions. Think how a wordpress blog with product reviews looks and then look at reviews.cnet.com and you will understand what I mean.