So basically, because default Drupal's database schema is too normalized and generic you experience problems.

As for Postgresql - you are going to write new code anyway - either in Rails or patch Drupal modules to work correctly with Postgresql. So this one is definitely not a deal breaker.

I would advise also to look into option of upgrading to Drupal 7 and using its DB stuff. While it is still join-heavy by default, you have more power to manipulate schema and for example to have flat tables for your content types or store some stuff in MongoDB. And all of it with still Drupal's flexibility and mostly configuring stuff instead of writing it.