The laws of ratios pretty much control these questions. With the same gear size the differences of a multy speed system in the same ratio as a single speed are far smaller then one thinks. Portions of a % point of efficiencies, depending on pulley friction and chain lines. The differences are far more perceived then real. Of course when your in the final sprint for the gold medal any minor difference counts. Are you doing this? The chain tension of your single speed set up will have a larger influence then the der tension/pulley wheels. BTW you say that the frame is made for single speed set up. The vast majority of forward facing drop out bikes I've seen/sold/worked on were not thought as a conversion possibility, back in the day. Can you explain more? Andy.