I'm not sure that the difference is worth worrying about. I have a 300 and a two 300S's, and there is no discernible difference in their firing behavior. The condition of the spring and seals is really a much more significant factor in the gun's feel. Frankly the 300 has always been my favorite of the three.
For what it's worth, Jim Maccari's superb replacement spring for this series of guns is just that--a single spring, which perhaps tells ya his opinion of the benefits of a double spring!
Historically speaking, the idea of double counter-wound springs is not new--the first application I know of, was on the classic BSA underlevers more than 100 years ago!
In more recent times, the Walther LGV used a single spring, but the succeeding LGV Spezial went to short counter-wound double ones. I own an example of each with a fresh rebuild, and again feel no usable difference in their behavior, in spite of their being smaller, lighter rifles than the FWB's, and without any kind of recoil compensations mechanism.
