What kind of Seven do you have? What kind of tires are on it - sounds more like they are breaking loose rather than getting airborne
Mid-engine caars intentionally have a low polar moment of inertia which makes them less resistant to direction change and easier to maneuver: http://www.autozine.org/technical_school/handling/tech_handling_4.htm
My steering does get lighter as the speeds pass about 70 mph. While it was a bit disconcerting initially, I no longer even notice it.