What make linings did you use? My series 3 stops very well with "Mintex" lined shoes.
I have just had the hubs and backplates for my 1929 BSA bike blasted and powder coated in satin black, well pleased with the result.
It was most likely me who recommended Mintex, so much better than the usual stuff.
Only 10" on the rear of my series 3, it stops very well when needed. Lots of engine braking with the 2.25 diesel gives the brakes an easy life.