I remember the Spence Wolfe brake boosters well. They came into the LBS where I worked pretty often. The Cupertino bikes usually had them, often along with a custom "alpine" jockey cage Campy NR RD, Super Champ 58 rims with rim ropes (always!), and tied and soldered spokes.
I found these online. Whoever is running the current version of the Cupertino bike shop apparently sells repro brake boosters. Obviously you could make your own.
Spence Wolf / Cupertino Bike Shop reproduction MAFAC brake booster plate - (NEW) - MAFAC caliper AFTER
FWIW a quick flex test comparing the new GC610 vs MAFAC racers shows that the former are dramatically stiffer. Therefore I'd assume that boosters would do more for MAFAC performance than for Weinmann/DC/GC brakes.
It would be interesting to see how they compare to vintage 60s Weinmanns. I'm not sure if the hearsay about them being based on them is actually true. Might be. Vernier calipers would tell the tale.