Correct me if I'm wrong, but can't you avoid compiling in production if you package everything as part of your CI build? Rather than deploy off of a git clone, you compile at the end of CI, vendorize your gems, generate a tarball, and push that up.
This minimizes production dependencies (including the need to compile gems live) as long as your CI environment sufficiently resembles prod.
That could be one solution, however there are some problems that spring to mind like deploying to different architectures (common if you're using EC2) - I guess this fits your point about your CI env resembling prod.
This minimizes production dependencies (including the need to compile gems live) as long as your CI environment sufficiently resembles prod.