We discussed the C4's, and what I understood was that the manufacturers engineer the fins for their best possible performance.
Yes, some fins include the rails just as some Jeep 4x4 vehicles include big, rugged tires, but that doesn't mean you should add giant all-terrain tires to your little Nissan economy car and expect better performance. Other aspects of the little cars performance will diminish greatly, like how well it handles going around corners.
The same applies to the fins. If we add rails we degrade the performance of our manuverability, but thrust improves. From what Mark explained, the trade off is not worth it, since all aspects of the fins are considered during design and testing before they go into production.
Now here is the catch, ........if you plan on freediving competitively, and all you could use were a pair of 2000HF, then it would be wise to add the rails, but fundementally you would never use 2000HF competitively. Instead you would use something like the C4 fins that were designed for that type of use and already have rails (like why a race car has a rear spoiler but a pick up truck doesn't).
If we add the rails to our 2000HF fins, we best be prepared to sacrifice sculling performance. I don't think I want to do that since I use my 2000HF for the purpose of general <20M playing around.
So the big questions are these, "Could we add rails to improve performance?" and "Should we add rails to improve performance?"
Could we: Sure, if we want to sacrifice overall performance of the fins, limit them to a use that they were not designed for, and thus never truly excel at.
Should we: No. We should choose to own more than one pair of fins of which each is designed for a specific type of use, and thus not try to engineer our own.
Use your jeep 4x4 with big tires for gripping the loose muddy terrian, use your Porshe for the highway at 200km per hour, and your Maybach for taking 4 of your friends in total relaxing comfort to the fancy restraunt.