Thanks for the replies. It sounds like the slack isn't really an issue.

I actually just received an email reply from Arkel (they're fast!) about it, and they stated that my bag is a slightly newer design, intended to reduce the chance of an ubuckled strap getting caught in the spokes. This is interesting, because, on the Arkel site it shows the older lash tabs and states "the lashtabs on the sides ensure that the straps will not dangle dangerously close to the spokes." So, I wonder if this was still a problem? (Do you veteran Buggers have horror stories about this?!)

Anyway, regarding the saggy helmet holder, they kindly provided the suggestion of folding the helmet holder back, like this:





It seemed a bit odd, but I tried it and it works, making the bag more streamlined in pannier or backpack mode. The straps still function to compress the bag as well (the straps get a half-twist in them, but you can simply pull the buckle off and flip it if you want the straps to be flat - works either way, though).

BTW, the Arkel people are great. Their rep has consistently been helpful, courteous, and prompt in answering all of my question. Excellent product and service.