What your CPU can handle and what your motherboard can supply are two unrelated matters.
The electronics on your motherboard are designed to handle a maximum of 16GB if that is specified by System76.
An anology: If you have a "walking" water sprinkler that will handle 32 gallons per minute and a pump that will only supply 16 gpm, the best you can hope for is a throughput of 16 gallons per minute -- unless you buy a 32 gpm pump.
The mobo, in this case, is your pump. It can only handle 16GB of RAM. It doesn't matter that your CPU, or sprinkler, can manage 32GB of RAM.
Your motherboard is the limiting factor. If System76's literature indicates that the maximum RAM is 16GB, then it is reasonable to assume that each of the slots will have a maximum of 8GB, unless a 16GB module will work in ONE slot. It wouldn't do any good to put 16GB in each slot, since it just wouldn't get used -- or worse. I'd go with what System76 gives as the maximum.
Hope that made sense!