How did you measure it? I mean, do you have any kind of storage on the line, like a Redstone Storage Cell? If you just replaced them, turned on the engines and ran to your machines, yes, you're going to see much less power. Why? Because the Redstone Energy Conduits are also mini-batteries. Each holds some amount of MJ, about 1000 if I can believe the Oracle of Google. Anyway, until your conduits fill up only a part of the power will get to your machines. The flip side is that if you suffer an interruption anywhere along the line your machines will still chug away as they are being fed from the excess power in the conduits. This allows you to make adjustments to the network on the fly without suffering a loss of production.
As for the energy loss it is time based per block, not distance based like other power networks like IC2, RP2 and BC3.