The fact that FAP/14 + 55 does not match dps of a weapon with the same iLevel does not mean that our damage is scaling badly and/or that with "proper" amount of FAP we would scale like other classes. You have to consider abilities and their scaling too. For example: while our finishers scale with AP just like rogue's finishers do, we get more from it because we have a lot more AP than rogues. Does that mean that we scale better than rogues? It does not because it's not that simple
To be honest I am quite satisfied with cat's damage output. It looks really good on pre SSC level. Perhaps someone with better gear can tell us if other classes outscale us in SSC/Eye and beyond.