Ruby and Rails feel best as a prototyping platform for getting something out the door fast, proving a concept, and not worrying so much about correctness or maintenance.
I don't think if you are doing a lot of TDD and larger systems that piling on more Ruby and Rails is the right answer. I think once you know what you are working with, a well designed language with a compiler is a huge help and would remove a ton of useless tests and stuff that you end up writing in Ruby by hand.
This very likely leads to an API driven system with an API written in a strongly typed, compiled language like C#, Java, Scala, Haskell, or Go and writing your front end in whatever makes the most sense for your team.
At that point you get the benefits of a nice rapid development platform like Rails for your front end, and a fast, testable, API written in something else using all the clean code architecture you want.
The trick is, you do everything ugly in Rails or PHP or whatever in your initial prototype and you might not even write tests. You just ship working code until your prototype has proven a business case. Then, you move it towards something with lower TCO over time. Make the investment in your code when it makes sense to invest in it and use the best tool for the job at each step.
You probably never need to leave the standard Rails MVC stuff on most projects unless they are wildly successful and/your long term needs change. Even then, you can probably keep the rails front end stuff and just talk to an API and be very happy.