Hi Morg,
You haven't shared your style of diving yet. These probably aren't bad for surface to moderate depth diving. I wouldn't take them very deep, though.
I will share that I started with Omer Millenium fins (plastic with full foot pocket) and they were great above about 75ft. I got well below 120ft once and the lack of performance of a plastic fin was quite noticeable as my ascent was significantly hampered by the blade overflexing when I tried to stereo kick. I corrected by purchasing a pair of Special Fins CarbonFiber Pros and now have all the fin I need for the ranges I operate in.
www.specialfins.com
The difference between a carbon fiber longfin and any other is like night and day. That isn't to say that other fins, like the Garas or some of the Mares dual use fins, can't perform because they do. Martin Stepanek and others have done record dives with the Garas as well as Special Fins and Spetton blades. The Mares and your split fins are not deep water fins, though. Put on a Spetton C4 or a Special Fin that fits well and is calibrated to your strength level and you will be quite satisfied with the improvement in performance.
Regarding footpockets, a full footpocket with an appropriate neoprene sock bootie (3-5mm) to make it nice and comfortable and a fin keeper to insure that it stays on when your neoprene compresses at depth is probably a better choice. Get your feet sized correctly, most finpockets are in European sizes. The Omers come in two size increments so when you try them on, they will either fit like a glove or they won't. Better if they don't, you can use the sock bootie to compensate and fit perfectly. When I order fins and footpockets I order two sizes of neoprene sock (3 and 5mm) and either keep it or use it for the odd sized gear set to get a friend going (more people in the water, more choices for the sharks!)
The really cool thing about it is that if you purchase Omer footpockets, you can use all kinds of different blades with it. Spettons and Special Fins both use Omer footpockets as do many others. You can simply purchase blades to insert in the footpockets. Pretty nifty. And an inexpensive way to upgrade. I bought my Omer Milleniums as a set and purchased only the Special Fins blades and saved a hundred bucks or so.
As I recall, one of the Scuba supply places was offering a $199 package for a pair of Omer Milleniums, sock booties, snorkel and an Omer Alien or Abyss mask (all pretty good stuff for the guy that is breaking into freediving).
Because freediving is so organic and fluid compared to tank diving, it is always to good to think about how your equipment attaches to you. It should be like a comfortable glove and give you ultimate control in the water. No looseness, no tightness, no discomfort.
Lungfish