minuscule it is and more drag = less fuel economy.
You're only testing it by pushing it on a bench (no load), testing it while applying loads transferred via the drive train under load from the motor is a different thing.
Aluminum pulley may last a little longer, but the pulley teeth sharpen into a pointed and will cut the belt. The money that you spent on aluminum pulleys could be spent on buying 5 bag of plastic ones.
I'm sure if Robert found a front tensioner to be beneficial it would of been added to the MRX5