The trick with a heavily loaded model like that is to be sure to avoid it lifting off early and be very careful not to let it pitch up right from liftoff to a high degree. You want to lift it up a few inches and sort of hold it there while the speed builds and only then let it "glide uphill" with a shallow angle of climb. When you're about 300 to 400 feet up then start playing with it. Up to then it's very gentle turns only. Once up to height start to wring it out in stages and be watching for any bad signs of early stalling and stuff like that. In particular it probably won't like tight turns or tight looping maneuvers since the G load will tend to make it stall in the tighter turns more easily when the need for lift ends up with you reaching the stall angle a lot easier than a light model would do.