Speaking from personal experience, that rear frame section is not the most rigid structure to begin with. Removing the diagonal braces is going to make it far less rigid. However, from the photos posted above what I'm more concerned with is the bracing that was added to the forward fuel tank supports. Those are designed to be able to yield forward in the event of a minor rear-end collision. By adding rigidity to those posts you've made it so that a rear-end collision will crush the fuel tank rather than allow those posts to yield and allow the tank to shift forward.