I've heard the standard glide ratio is around 2:1, with really good fliers attaining 3:1 or more. I've seen claims of wings that will yield as much as 5:1, but those seem to be outliers. (Google wingsuit glide ratio to see the full spectrum of opinion)
http://www.flylikebrick.com/articles/flb_fallrate_vs_glideratio.pdfAs for how slow you can fly, I've heard the critical window is pretty damn narrow, with most reporting optimum glide speed somewhere in the low 70s.
(I don't know any of this from personal experience, and am happy to be corrected by anyone who actually knows what they're talking about.

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