The Supersport has a lined piston which should cut down quite a bit of twang, BSA spring guides are usually rubbish as they are made to acommodate a wide variety of spring internal diameters.

I usually make up an extended guide in brass to fit the spring concerned, that and a little polishing of the piston skirt and honing the cylinder will give you a silky smooth action.

A bit of work and a relube will make all the difference.

Rich