Look at it on the flip side, if a customer even bothers to sign up to a community they must be semi serious (its hard even getting people click off Facebook).
Once on they will be wowed that there is an APP and straight off the bat they won’t forget as much about the site as just using a browser.
In my market having an APP is something to be proud of and shown off, lets be honest the way you guys present the link to the app is not intrusive (most sites are plastered in banners).
I am pretty certain having the app available from the start will increase every Discourse communities conversions.