And perhaps I was lucky, but typically readability and maintainability are orthogonal to performance and efficiency. Sometimes readable will have optimal performance, sometimes not. Then it becomes a matter of tradeoffs.
The first entry even stated that they were guidelines and if you have a valid reason to deviate: discuss it and you'll get an exception.
The discussion thing was mainly for new coders. We had a library part of the code that was used by many programs. Optimizing parts of it for their use case, could make it unusable for the others who used it.
Communication is key. If you discuss, before implementing it, why you're making certain design decisions then everything goes a lot smoother. If there are objections, keep in mind that the worst case isn't throwing away your design and starting over. The worst case is implementing it and screwing over your fellow coders.