I have never seen any direct evidence to support a direct ranking boost from quality outbound linking.
A few years ago I did an experiment. All the outbound links on the site were through an internal tracking redirect that was listed in robots.txt. Googlebot had no way of knowing that the links were external. We changed half the pages on the site to use a different tracking redirect that was not blocked by robots.txt. After a couple months, we saw no ranking difference between the pages that linked in a way that Googlebot could see vs the others. We also saw no ranking changes for the site as a whole. We did measure the position in the SERPs of the pages that we were linking to. For the pages that we linked to where Google could now see our link, rankings did rise modestly.
Personally, I think that Google has made a big mistake by not giving sites with quality outbound links a modest and distinct ranking advantage. Google relies on links to measure the quality of pages on the internet. Google should be providing an incentive to webmasters to recommend external resources. Having not done so, their algorithm is no longer as relevant or effective as it could be.
On the other hand, there are almost certainly indirect ranking boosts from linking externally when it pleases the user. When users click on external links on your site, doing so may prevent them from using the back button to return to the SERPs. Google measures the "bounce back" rate of users returning from your site to the SERPs and uses that as a strong signal that influences your rankings (particularly on a page by page and query by query basis). The experiment that we performed would not have measured any ranking changes from this because links were available to users in both cases.