Experience from over 10 clients completely switching their sites from http to https (especially online shops): In terms of ranking, it does almost zero. On the other hand, there are loads of small details that can make the switch bumpy, you especially want to make sure proper 301s http -> https are in place (yes, it's like switching domains!) and if SC (GWT) are used, the new property is defined as preferred. You need a proper certificate and proper encryption (some don't meet Google's minimum requirements), you need to make sure external resources are integrated via https...it's not THAT easy if you want to do it right.
The days of the free SSL services are counted anyway in my opinion, many of them are massively abused by phishing sites.
Maybe https will positively affect rankings in the future, but at the moment it's just a nice little green lock that may help in trust with visitors sensible to security. Whoever claims SSL improves rankings didn't test it personally and just repeates what Google is saying. If you are planning to switch to a new CMS, do a relaunch, restructure the whole website or similar, I'd recommend to get https as well, won't hurt, but it's far from mandatory to get excellent rankings at the moment.
As far as the lower value of links via a 301 is concerned: Yes, there is a slight difference, but it's not worth mentioning. You won't drop from #3 to #5 because of a 301.