Raideron, I figured the some thing in my model.

The firewall have a little movement behind the holes in horizontal pressure, but I don't saw nothing with vertical pressure!

Anyway, I think this condition in flight almost never happen and is not a problem.

Maybe theres a little - or none - of those vertical and horizontal movements in flight because the engine pulls the airplane vectorially and the force is distributed of the mount for firewall and it for fuselage.

It is only a consequence of light structure.

The problem is not the firewall crack, but it to unglue of model.

Theres a pic in that forum of a RCU member what had him firewall unglued!!