My testing has shown:
<1200g and it performs best.
at 1300g it is sluggish and clearly shows it is heavy.
at 1400g it is clearly at or above it's usable weight. It will struggle to climb and more importantly it will really struggle to overcome a rapid descent.
The most I ever tried was 1500g and it did take off, but required 100% thrust 100% of the time to do anything, even yawing (at 100%) would lose altitude.