ya I have a little gy6 chinnese scoot and that thing moves faster then my kymco.I think its because of the hefy frame work and the tires on the kymco slows it down some.
The kymco agility is a quite heavy bike, yes. Its 97 kg dry weight, so plus 5L fuel and you, its between 160 and 200 kg. And as far as i recall, the max weight for any gy6 engine is 2 - 300 kg (might be 225 kg for the agility, but i cant document my sources.)
The added weight for the same engine can in some cases matter, but if you derestrict and optimize it, there shouldnt be a big diffrence. My agility can outrun many derestricted 2-strokes in both accelleration and top speed.