John – think of it as a tiebreaker. If your site and one other site are essentially equal in ranking weight, but you have an HTTPS site and they have an HTTP site, you would win, and rank first. Bottom line, it’s a weak factor.

John, so far the people that have tested this have not been able to discern any sizable ranking boost.

That is definitely the big question in most peoples’ minds. Here’s the way I see it. Yes, traditionally a 301 redirect does introduce some loss of PageRank. Plus, potentially slower performance could have this impact too. But Google really won’t want to be in a position of telling people this will boost their rank, only to be faced with a bunch of unhappy people who follow this advice and lose traffic. So I think they’ll either tune the HTTPS boost to offset those factors (leaving the effect roughly neutral), or will do something to recognize this configuration and minimize the normal redirect “cost” so that you end up net (slightly) positive in terms of ranking signals.
I do think it’s still early days, and Google may still be working out some kinks. We also don’t have much in the way of case studies to go on. So individual mileage may vary! I’d say those who are risk averse may want to wait until there’s more experience to go on, or test this on a small site, a mobile site, or a section of the site before rolling it out on the main site.

Glad to hear the wp-engine solution got you that speed boost. That’s pretty substantial, and should be a positive for your users, and possibly for rankings as well. Especially beneficial if you have mobile users!