In the 1960's I used to own a French Peugeot UO8 with steel rims that stopped just fine when wet. The secret was that the braking surfaces on the sides of the rims had grooves in them to dissipate the water. Also, the brake blocks on the Mafac center pull brakes were huge and the leverage was tremendous. The real problem was that the brakes were too strong in dry conditions, and I endoed the bike several times when I grabbed the front brake too hard. I had a friend who owned a Schwinn Varsity with smooth sided steel rims, and he wrecked out a lot in the rain.