The stage 3 Roush is putting down about 380 HP, they have a better suspension (for corners anyway, better matched shock/struts, springs and sway bars), and better tires.
To beat a stage three you're going to need some help, mainly in the power and tires department. The off-road H isn't a bad idea, since you can swap the stocker back on for inspections, only takes an hour or so. Next I would do LCAs and sticky tires.
For future mods, I would suggest two basic options a supercharger (I would go with a twin screw), that would put you well past the stage 3 in terms of power, or heads cams and possibly the new C&L intake coming out, that should put you around the power level of the stage 3 but since they have a roots blower they will probably have a better power curve.
Dan