That is odd. I've done at four conversions from 700c to 650b using two Tektro R559's, a cheapo Chinese knockoff, and Mafac Raids. All have sufficient stopping power and are drop bar bikes. Two have aero levers, one has standard Suntour Superbe levers and the third has Dia Compe Guidonnet levers. The Guidonnets are not as good, but I believe that is a function of the available leverage, not something inherent in the bike as I my friend has similar braking on his standard reach brakes with the same levers.