Should I expect adding more RAM of the same model to lead to worse benchmarks?

Solution 1:

I bought 4 2x16GB Corsair Vengeance 3200 LPX kits thinking this would be the same as buying 1 128GB kit.

Considering the Corsair Vengeance 3200 modules in the 2x16 GB kit and the 128 GB kit are identical your assumption was correct.

When I was running benchmarks with Passmark, I discovered my RAM read/write speeds and latency got nontrivially worse as I added more RAM. With dual channel 32GB, I was on the high end of AMD machines for that RAM model (2400 PassMark score); for quad channel 64GB and 128GB, the low end (sub 2100). I was a little surprised by this.

You should verify you were not in a mixed memory channel configuration. In reality outside of benchmarks, you will see little improvement between small frequencies differences.

I'm trying to decide if I should return it and get a 128GB kit (e.g. Vengeance LPX 3000).

You are welcome to do that, but there isn't a performance difference, considering the physical modules are identical. The only difference would be the serial numbers would be incremental with one another.

Could it be related to the fact that I'm running 4x32GB kits as a 128GB one? (i.e. are larger kits fundamentally engineered differently? I had assumed the differences were primarily packaging and guarantees they were from the same manufacturing batch)

The only differences are indeed packaging, manufacturing batch, and price. It is basically the difference between the huge bag of potato chips, or the combo pack of potato chips, which contains two smaller bags.

Are kits and motherboard manufacturer compatibility charts primarily just guarantee the RAM works at all, or are there likely nontrivial performance gains to be had with a "correct"/supported RAM setup?

The compatibility list is simply the list of modules that were tested. It should be used as a guideline to find modules that are compatible (compatible frequency for example). DDR4 is a standard, so a module running at a certain frequency and timing is going to perform nearly identical to another similar DDR4 module. Other properties of the module that are not part of the standard (what type of heatsink it might have) are the only major differences between modules produced from difference companies.