<<concerned that I'll put way more microballoons on than necessary and just add weight. I guess the answer to that is to just keep sanding it down>>
If it takes a lot of filler to get the contour you want, perhaps you should review the underlying layup or the contour of the substructure. If you gotta add a lot thickness, let it be glass/epoxy, not filler.
That said, ripping off excess micro is no problem if you have the right tools. You need some sanding boards and a vixen file.
Pick up a 24x24 section of 3/4" birch plywood at the home improvement store. Rip it into boards 17.5 x 2.75. Visit the local auto body supply and buy a few "sanding strips" in 40, 80, 100, and 120 grit. They're normally used on an inline air sander. Use 3M spray adhesive and glue them to your boards. Now you have large abrasive files that allow long two-handed strokes across the contour (shaping) or serve to flatten when used along the contour. 40 and 80 will rip down micro quickly. Use the 100 as you get close to finish contour and 120 to work the surface flat toward the end.
Concave surfaces are done the same way. Use the 3M adhesive to attach good sandpaper to any kind of hard tube (PVC pipe is good) with a radius a little less than the part you're working. Use some rolling motion as you stroke.
Everybody working aluminum and glass should own a sharp 12" vixen file:
http://www.cleavelandtool.com/catalog/WEBfv8.JPG
Hold it two-handed and work it across the surface at an angle to it's length.
Avoid "finger sanding" (holding the sandpaper in your bare hand) like the plague. No way you can get a non-wavy surface with bare-handed sanding. Use hard wood blocks, flexible rubber blocks, foam blocks, whatever.