I ride SS and geared, started SS about 3 years ago. It made a huge difference to my riding, as some say above it really has taught me momentum and how to attack a hill – something I do on geared now as well without having to think about it.
I notice that some other riders in out night group get to the bottom of a hill and then are fiddling with gears and clashing chain etc until they are at walking pace, then grind up slowly. Whilst they don’t need to do that, quite a few do and that is the difference from riding SS – you cannot afford to slow down in that way. They are harder work on a long flattish road section, but then the high speed spin burns calories.
My riding improved significantly from starting SS – don’t know how much is fitness and how much technique, though.