The heavier the plane, the higher the stall speed. The store page indicates it should have a dry mass of around 155g, so it sounds like your on the order of 2x the expected dry mass.
So it is going to need to be going really fast to stay air born.
I see an EST. stall speed of 20mph (33km/h) at 350g (which your going to hit or higher, higher will just bring the stall speed higher)
Where as at the documented weight, I see an EST. stall speed of 16mph (25km/h) @ 200g (155g dry + approx battery weight)
Given the size of the plane, I would want the stall speed to be on the low side as a football field... maybe 2 is about as far away as you would want to fly that and still be able to have good orientation viability.