Brake Upgrade

I have an 04 XE. I tow an open hauler with a 350z. Plus tools, jacks and couple set of wheels/tires i'm towing about 6,000#. I noticed brakes felt a little weak. Some people were saying the 08+ brakes are redesigned? How much would that cost to upgrade? what are differences and part numbers? I was thinking to just change to steel braided lines and better pads and fluid.