hi panama,
Welcome!
all else being equal, practically any luxeon-based flashlight (or mod) will have its brightness most affected by the *binning* of the white luxeon LED(s) used.
according to the luxeon datasheets, the differences in nominal output between the dimmest and brightest bins can be up to 100%! and that's not even mentioning the differences in color temperature and forward voltage!
any small-to-medium volume manufacturer using luxeons will very well know also that procuring the luxeons themselves in specified bins and qtys is a tricky affair and is very much the luck of the draw. even if you buy them by the 1000s, you can't always guaranteee that you'll be getting what you want or that you'll even be getting all of the same thing (you might end up with mixed bag of LED bin grades)!
all these factors conspire to make life a living hell for poor guys like Peter Gransee, who has to consider the possiblity of waiting for WEEKS for another shipment of parts if he rejects what he gets. and even then, he won't be able to trust what he'll be getting as replacements either!
recent batches of luxeons have been getting brigher and better in general, so its not surprising that newer revs have more oomph.
and if you examine the datasheets, you'll notice that the HDs and LDs have identical luminous flux specs. trolling the threads will also yield further proof that the LD luxeons have as much kicka$$ potential as the HDs if properly applied.
IMHO, you didn't make a mistake. The Arc LS is a fine flashlight and it was unfortunately and unevenly affected by one of the few things beyond its manufacturer's control.