So I've noticed how sensitive the Crosstrek is to added weight in the rear. I've done a 600 mile trip with 4 people and luggage and recently hauled about 320 LBS of Quickcrete in the back. Both times the car felt a little squirely and tended to want to oversteer. Gentle inputs were definitely required. What would be the best least expensive upgrade or fix? Adding a few extra pounds of air in the rear tires which helped but should I be looking at sway bar upgrades? Ideas?