Dan,
Stumpy's reply is a good template. The best way to start is to make sure the cables are new and big (resistant to flex). All the mods are beneficial, but do the cheap improvements first to see if you are happy. A good brake lining is important. Old ones tend to be hard and slippery.
Clever Trevor discovered that the sponginess in the brakes may be due to a geometry error. The various attemps to rectify this is truing the brake linings on a lathe, the Burt Weiz floating pins mod in MPH and the Tim Kingham mentioned mod. This last mod was a favorite here in the States used Rick Voshel and Marv Kummet. They would file the axle hole in both brake plates so it was a little large. When they installed the wheel they would leave the hollow axle nuts a little loose. With the wheel still in the air they would hold the brake lever tight causing the shoes to align the brake where it should be. They would use a special thin wrench to tightn the hollow axle nuts while the brakes were on. The wrench is now made by Niel Videan in Australia (see adver in MPH).
There are three popular upgrades. The 8 inch brakes, the 4 leading shoe conversion and disc brake conversion. The first two are easily available the third you will need to do yourself. I have been told the 4 LS conversion in quite good. I use the 8 inch brakes as 4LS brakes are illegal in the racing rules I have to comply with. I find them quite good on the track. If you have Rapide drums you should consider new Black Shadow drums if you are doing the 4LS conversion. I have shattered Rapide drums.
Finally, if you are running stock springs on the front end your stopping distances will be longer because the front end does not go up and down very easily during braking. Glen's video seems to show this as his tire flattens so much. Even with low tire pressures the front springs should be compressing as the weight transfers to the front end but because the forks are seizing the springs do not compress, the tire does. Very impressive!
David