I would always use Gush as a PHAR. But you raise an interesting point - if people are going to install Gush in their projects with composer then requiring 5.4 might be a problem - but I don't think that would be a main use case.

My only problem is context switching, i.e. when working on Gush I would need to remember to switch to the Gush standard instead of using the "normal" standard.