I'm trying to edit the shipped speeder bike, but when I convert it with rep's mesh importer and open the .obj there's all this extra nonsense. A couple spheres and a siamese twin-speeder. What is that and how do I make it go away?

Assuming you mean the regular speederbike and not the Barc speeder, those 2 large spheres are collision primitives and the smaller one is p_crithit, the siamese twins you speak of are the lowrez and collision geometry, there might also be some small geometry where the hp_fire, hp_active, and bone_root are located.

You can get rid of these very quickly by selecting the model and welding it with a tolerance of .001, then activate the polygon island selection filter and select the parts that you don't want with a single click and delete them.

I would recommend that you extract delete the lowrez and collision geometry so you can use them again later, then replace the small geometry that you deleted with nulls because you will also need them, be sure to rename everything the way it was originally too or else you'll need to edit the odf.

You'll also need to restore the bones so the stock animations will still work or else you'll have to create your own, and if you figure out how to get custom FinAnimations set up correctly in XSI then be sure to write us a nice tutorial.

You shouldn't even need to weld the original model; you just select a face on the model you want to delete and expand the selection with Shift +.

Shift++ or "Grow Selection" only works efficiently when polygons are connected, hence the reason to weld the object in preparation for editing, when you use shift++ on disconnected polygons the process becomes more difficult than it really is.

Edit
From the time it was imported to the time it was stripped down to the main mesh took less than 3 minutes with very little effort, I don't see a reason why anyone else couldn't do the same: