I've been doing ruby on rails for quite sometime. The 80/20 problem is not unique for rails. It applies generally to the entire world. I'm also not aware of any framework that can just do business logic for you.
To answer your specific questions. Conventions will not stand in your way while doing the 20%. Instead, conventions will help you to get through that 20% quicker.
Personally, for user authentication, I use Authlogic. For user authorization, I use Lockdown or Authorization plugin depending on customer need.
I also use inherited_resource in most of my projects to simplify controller code. This is another power of convention.
To increase speed of development, you will not only need to know Rails, but rails gems/plugins that does the right things for you, so you don't have to reinvent the wheels again. Also, knowing Ruby language is a must for quickly develop beyond the 80%.
Ruby Toolbox provides some of the most popular gems and plugins used in typical rails projects targeted at specific domains. You can look through the relevant categories and know what most people use. (And it's probably a good idea to use popular, well maintained gems)
TDD/BDD style development will also help you to speed up in the long run.
Lastly, a warning: If you stray away from rails convention, you will have a painful time in general.
P.S. I used Merb before. My feeling is that conventions helps you in merb, but you won't get too much penalty for not following them in merb. However, my experience with Rails is that if you decide not to follow rails convention while developing rails app, it will come back to bite you in one way or another! So think twice when you really attempted to stray away from rails conventions... (This is from my own experience, and of course subjective, but you can think of it as a warning...)