I guess its a neat idea to use a ULA belt on a GG pack, but if I needed better load support for heavier loads, I'd probably just get a ULA Circuit or Catalyst pack, which although heavier than the Mariposa plus, it would also carry 30-35lbs with better comfort overall, IMHO, anyway.
These Ultra-lightweight packs are great, but I think if I were ever lucky enough to be able to pull off a Thru-Hike, I'd be using my McHale LBP36, close second would be a ULA Catalyst followed by the Circuit, then a GG Mariposa Plus, and possibly a Granite Gear Nimbus Ozone. All of these packs share one common trait, in that the hip-belts are all interchangeable, So when and "if" you shed those extra pounds, you could go with a smaller hip-belt. The Nimbus Ozone stands alone here as the only pack with interchangeable shoulder straps. Also with the McHale pack, you would need to have a smaller hip-belt pre-made before your trip and ship it ahead as needed.