if you're a DIR practitioner, keep in mind that you need heavy fins to offset the front heavy nature of backmounted rigs. Especially true if you're in doubles in a drysuit. F1's have two design factors that make them the best fins for backmount. They're heavy as hell, and they're actually just a really good design. Unfortunately, that first part makes them one of the worst fins for sidemount. Hollis is committed to sidemount and is focusing on that, and realize that while they are one of the larger sidemount rig manufacturers, they are selling basically no fins to that market because they suck really bad from a trim perspective. To fix that, they have to make them neutrally buoyant, and that means that while they certainly can be used for backmount, they are far from ideal.
If you need a neutrally buoyant paddle fin now, the OMS Slipstreams are the best out there for the traditional design, though I much prefer the Dive Rite XT's. The Deep 6 fins will be out soon and those should be better than the Slipstreams. Unfortunately I won't be able to dive them because they aren't coming out with a XXL for a little while, and I can only get my feet into the XL's when barefoot, same with the Jetfins and Slipstreams