A quick note on the AC3D plugin based on a conversation wtih Tom....
The AC3D X-Plane OBJ export plugin is not an optimizing plugin - it is meant for "finishing".
The idea is that the plugin can edit an OBJ and preserve almost any strange sequence of attributes you come up with. If you've carefully ordered your polygons and used ATTR_no_blend and a bunch of other tricks to use translucency and other tricks, the plugin will not scramble your work, but will preserve it. This lets you put the finishing touches on your OBJ, hence a "finishing" plugin.
The other side of this is that in preserving your object, the plugin does not attempt to make any optimizations that would reorder your object. So if you "ask" the plugin to make an inefficient object by using lots of state changes, it will do so and your framerate will suffer.
Basicaslly there is a direct correspondence between the model in AC3D and the exporter OBJ file. So to use AC3D efficiently you must understand both what makes an OBJ fast (not using ATTRibutes among other things) and how you do these things in AC3D (not using flat shading at all, for example).
The best of both worlds would be of course to have a menu option to optimize a model, which would let you optionally change polygon order for speed. This is something I would like to do but I'm not sure when I'll have time to code it.