bought this along with a load of harley benton 11 and 10 gauge strings, obviously, switching from 9s to 11s on a stratocaster you usually need at least one extra spring. These are very cheap, they aren't the same length as an american deluxe stratocaster springs, so if you are planning on mixing these with standard american stratocaster springs then make sure to have them set up symmetrically, ie don't have one of these springs on one side and a standard on the other. Have this one central, or buy two and have these at the edges and put the standard ones in the middle.
Also noticed now my guitar reverberates a lot more when unplugged, the 4 springs act like an on board reverb unit.
I don't believe springs affect tone, I do however believe that a rubbish one will affect your tuning stability, this is fine on an american deluxe stratocaster, no better or worse than the normal springs.