I've tried A/Z testing this with a few of my sites - removed all of the outbound links for one site's category pages, and kept the do follow external links on the other site's category pages. From what little empirical evidence I was able to observe, it appears the pages with do follow outbound links (highly relevant, too) performed better after 3 months.
Of course, this all depends on what you're linking to, its value, and the relevance to your content.
Also, from what I understand, creating no follow external links still cuts into your page's 'total link juice' - for example, say you have 10 links on a page, 5 internal, and 5 external. Now, you change 4 of the external links to 'no follow'. You would think that now, your internal links would get 5/6th the page's juice, and your 1 external would get 1/6th - not so fast, apparently, you're still only giving 6/10th to internal links, and 1/6th to the single do follow external link.
Feel free to correct me if I'm wrong.