Before adding additional suspension, I would look at preloading the existing more. With just additional preloading of my upper and lower overload springs, I was able to carry a heavy camper without increasing the spring rate. If you still need help additional help after preloading, that is good time to start looking for additional suspension support or swapping out your existing springs.
Although I do not have experience the Timbren and Sumo springs, I did experiment with a similar concept using Energy Suspension and Torklift upper StableLoad spring pads. The Energy Suspension pads compressed more giving a softer ride, but compressed too much for my use and didn't negate my sag. Your use of one these springs will depend not only on how much additional weight you want to suspend but how much clearance you have between the stop and the spring and how compressible it is under load.