Dear folks

                I am bit confused... Need some help to figure out.

Can I run a quadcopter or any multirotor at full throttle... 

Which means can I make a quad  whose total max thrust is equal  or Just a 100 grams above the total weight...

Rather than saying the extra weight in number it would be more meaningfull if you mention it in percentage of extra lift available.
Because quads are available in different sizes.
For a 180mm and above model 100gm is no good. For any quad it is always recommended to have weight half of available thrust.
This is because quad hovers at half throttle so when sideways the movement would be easy because you have plenty of power left.
I hope i am clear.
For doing aerial stunts 250 size quads pack almost 5~6 times thrust as that of weight so that we get sweet punch out of it.

Thank you sir...

But now the issue is... How about the motors produce their max efficiency at full throttle??

Motors efficiency is like inverted parabola it will increase at starting to some point and later on it will decrease.
Most of the data shows the same it stays constant upto certain level later on the thrust/watt starts to decline.
At full throttle they are not at all efficient.

That's right  sir... No motor provide max efficiency at full throttle...

But pls have a look at this thread sir
http://www.rcindia.org/electric-power/efficient-motors/

Hey i know this thread ad i put my comments on that already