Even with straight wing, going at near 700km/h is still possible. More to do with the small engine. American avenger is much bigger with a huge hunch back to accommodate a very big engine.Plus the design might not be suitable to take on higher speeds, especially the long and rather straight wings, no? What is the expected top speed, since 580km/h doesn't seem too bad, actually.
If the drone is to do with fast reaction unit. The speed to response to a further area seems critical.