Ask for the email permission on the Facebook acceptance dialog should not decrease conversion. Unlike many of the other permissions on Facebook, this one does not add an extra step. Nearly all apps ask that users provide their email addresses.
Each application's conversion ratings are vastly different as it depends on the brand that is backing it up. The Facebook login screen itself doesn't have the highest conversion rating, but that depends on the source. Conversions from the app center tend to be around 30% whereas the conversion from web tends to be around 60%.
Unlike in the past when users had the ability to check off the permissions the app got to see, now general profile information including public profile, email, friends, and likes are simply listed in a sentence. The other permissions are listed in additional authentication modal.
Of course beyond that anecdotal evidence is not something I can provide. But if you are using it as a sign up when you have other options for sign up that have email, your conversion rating shouldn't be affected quite as negatively.