yeah, there is a strong law of diminishing returns...
I think my advice would be to start off with something basic / cheap, and you may find that is all you need. Job done. If you want more, you will at least then be more educated in what exactly you are looking for having sorted out the basics (i.e. properly fitting spring guides / piston sleave / seal / lube) first, and you will know if the gun has the inherant accuracy to be worth the effort.. you can then go onto whatever floats your boat (e.g. need it softer, or faster, or less hold sensitive, or quieter to cock, or easier to cock ) - with a specific goal in mind, which will dictate your selection of tuner / kit.
Ignoring re-engineering projects, of which I've obviously done a few, I have to say a lot of my springers have still just got a basic respring / tighter guides / plastic piston sleave and are really quite nice, thank you. Even when a resleave or something else involving a lot of machining costs only my time, not £200+, I still won't just do it for no reason... if a gun shoots nice with a basic tune then job done
HTH - JB
PS Pete, the best BSAs are pre-war, and I guess the early airsporters are OK too
Always looking for any cheap, interesting, knackered "project" guns. Thanks, JB.