Helijah, You are not quite right!
We have to differ here between Objects; Meshes and Materials.
An Object is made of one or more mesh. A mesh is a grid of many vertices which shapes the Object. And material defines how each group of vertices may appear in color, specular and transparency. You can have multiple materials in one mesh.
As I understood todays GPU's they don't care much about meshes, and vertice numbers, but object numbers. So you should keep this small.
Even if using the materials animation to set the parameters in .xml, you need to split the aircraft into several objects, so you can apply different materials to it via .xml.
And splitting an object into several ones needs more perfomance, not really noticeable on on modern systems, but on older systems.
Edit: so better have indeed multimaterials in one mesh instead of many objects with just one material.
"Within your modeling tool, try work with large meshes instead of groups of small meshes - but please don't mistake this as a request for grouping multiple, distinct buildings into a single AC3D file. I know that this can result in a very unpleasant workflow; we are working on optimizations in FlightGear that will combine mesh parts automatically."
from:
http://wiki.flightgear.org/Howto:_Improve_framerates which includes the small guide from where I quoted, written by Tim Moore, one of FlightGear's graphis and OSG-specialists.
And to my knowledge the transparency ordering isn't needed with OSG any more. It was when we used plib, but that's long, long time ago.
Btw. why some of your aircraft are having window glasses which flashes together with the lights? (Aircrane, DR400...)
At the end it is apperance and details vs. perfomance.