Hey Thor,
Does HTTPS play a role in SEO
Yes, it does. At this stage, it is still more of a minor ranking factor, but Google are definitely factoring 'https' into their algorithm when ranking sites, and the strength of the signal is only expected to grow with upcoming algorithm updates, as Google is pushing quite hard for a fully encrypted web. e.g. expect an upcoming Chrome update to flag non-SSL enabled websites for users.
See:
https://webmasters.googleblog.com/2014/08/https-as-ranking-signal.html
http://motherboard.vice.com/read/go...ll-websites-that-are-unencrypted-chrome-https
Does a SSL certificate + EV (greenbar) play a bigger role than just a regular SSL certificate?
Tthere isn't an SEO benefit at this stage for EV certificates. The real benefit of having an EV certificate is to provide your customers with immediate & extra reassurance via the green bar. The extra vetting process required for issuing EV certificates coupled with the company name prevalent in the highly visible green address bar, means that your customers are immediately reassured that they're dealing with a legitimate company.
For example: Many people, rightly so, are still very hesitant to enter their credit card details on a checkout page, so having "Company Name (Pty) Ltd" proudly displayed in the address bar, goes a long way in establishing trust.
As it stands, a padlock coupled with an 'https' website address is still an indication of a website's security, but with the current push towards a fully https-enabled web, we can expect that to change, as ALL websites in the future will need to be SSL-enabled, even websites which are not deserving of trust. When that finally happens, a simple padlock on a website will no longer be a strong indication that the site you're using is to be trusted, as a standard domain validated certificate can be issued to pretty much anyone.
At that point, EV certificates will become 'the new norm' insofar as establishing which websites are hosted by legitimate companies & are to be trusted.
Bottom-line: If you're thinking long-term, having an SSL certificate on your website should be a fairly straight-forward decision. They are an established SEO ranking factor, they demonstrate you value your users' security and browsers WILL start flagging non-SSL websites in the near future. Plus, you can pick up a 1-year DV SSL from a reputable brand such as Comodo, RapidSSL or even Thawte, for a very affordable price.
E-commerce stores, large enterprises such as banking institutions, or any online business seeking to maximize customer confidence, should strongly consider the benefits of an EV certificate.