Again, I haven't received mine yet, but, just to chime in with some information on the subject, I ordered the standard belt. From my research before placing my order, it seems that some people need it due to body shape issues (reported as not having a butt), but otherwise, it mostly seems like this add-on was originally designed for older versions of the Blast, not the Haul, which used to have a suspension system that had thinner padding. The reports that I read indicated that the new suspension system has removed most of the need for the extra padding.
I tried on a Haul at the factory, but it was just for sizing. It didn't have any weight in the pack. It felt great, but that doesn't really say much, since the real test will be to try the pack maxed out with weight.