What exactly do you mean more metal for the engies? Trust me the version in the update isn't gonna be a renamed beta 3.
Well, as a dedicated engineer, I found that the metal is too small to support engineers, placed too far away from the spawn to support teams that are playing defensively, and placed out in the open where it is very easy to get picked off. This applies to all of the other classes as well (i.e. the heavy because he runs out of ammo fast, and there are only these two ammo packs to help him). If this wasn't changed in the Scout Update version, I will be sorely disappointed.
My logic with the current arena maps (Granary, Badlands, Well, Lumberyard, and Ravine) is that they have very little health, but ammo is placed in a lot of places that will as a result support sentry positions. In Lumberyard, the ammo pack directly outside of the spawn help engineers set up outside of the cp doors. The ammo pack inside of the cp building in Lumberyard also supports engineers setting up indoors. Both are full ammo packs to help engineers set up, and because players need ammo. I've noticed a lot of great-balanced maps come from people who play a lot of engineer, because they know where a sentry should be and where it shouldn't, and adjust ammo placement accordingly.