 The rear carrier system is the best place to carry luggage - it’s behind you so is in your slipstream, and can take very large panniers and a top bag or tent. I’ve carried 50kg+ on my rear carrier, though I wouldn’t suggest doing that often! It’s rated at 25kg.
For more carrying capacity, the lowrider carrier is the way to go - it can take another full-size set of panniers, up to 25kg, and with it’s low centre-of-gravity does not affect the handling like front panniers can on an upright bike. It’s also handy to be able to reach into the pannier when you’re riding to get sandwiches or a drink! The little propstand shown is a handy extra. If you don’t have the lowrider, another propstand is available which fits to the rear triangle. | 
The SpeedBag is another carrying option - it can hold 120 litres of luggage, and is very aerodynamic - it fits instead of the rear rack. The Streamer front fairing makes you even faster (and keeps you drier)!
