I am guessing you did not add any bump stops or anything to help with the sag? I was going to add some Timbren Bump Stops, but my Malibu boat and trailer is pretty close to your weight of 6400, maybe even less.Boat and trailer are around 6400#s. Tows way better than my gen 2 way less turbo lag off the line and lower rpm. Also has way less squat in rear (guessing because of the 5 link) just feels better overall towing and the active exhaust is just icing on the cake