I depends on quite a lot of little things.
First there is the weight repartition of your bike, think of it as a hammer : if the hammerhead is up high it will be difficult for you to vertically put it on a table in terme of stability but if the hammer upside is down it's not a problem. This is due to some lever phenomenon occuring and there is the same thing when you're on your bike.
It's mainly because even if you're light you're still the heavier part of it.
Now the thing with weight repartition is that's it's linked to efficiency because of the fact that it's your speed that makes the balance, the harder the balance to find the more energy you need.
This applies to every bike but the lighter you are the easier it is to have good weight repartition. (because 10kg+50kg is very not the same as 10kg + 80kg since you can't transfer your own weight)
So then again it's marginal but if your bike has good vertical weight repartition this should work for you and not against you and it's more easy to find.
Secondly there is the momentum issue, the higher you weight the less equilibrium you have but you get more momentum, that means that to a attain a good regime in some circumstances you will have to burn more energy under a certain weight. There also is an equilibrium to find because if you are too heavy it's the same thing, the momentum is harder to find.
So i'd say the lighter is not necessarily the absolute best, it depends on your general condition and gaining or losing weight depends on that. For the rest there still is some technical aerodynamics to do on you machine but I dout this alone would make you gain 10mph.