I see WG is selling single spring kits for the 300S that it is calling 'stark' (ie extra powerful), for export. It would be interesting to see what power they develop.

Maccari in the US sells a single 300S spring that makes more power than the standard double springs. I vaguely remember reading they produce 50fps or so more, but not more than that, without making the firing cycle any harsher.

Alistair (Hsing-ee) fitted a Maccari spring into one of my FWB 150s many years ago and it was a bit more powerful than before, but I think he had a tough time getting it into the cylinder.