I'm confused, can you explain more please?
I'll do my best.
The integrated memory controller (IMC) on Zen/Zen+ systems is somewhat limited in how it can handle high-density RAM and/or large numbers of DIMMs. There was a big deal made about this last year when Ryzen was brand new, but basically, you want "single sided" DIMMs for maximum memory clockspeed with tight timings at those speeds. Actually what you want are DIMMs with minimum "ranks", it's generally assumed that single-sided DIMMs are also single-rank (which is not always the case)
Anyway, when it comes to modern DDR4 DIMMs of current density levels (8GB per DIMM or higher), 8GB DIMMs are often single-rank while 16GB DIMMS are dual-rank by necessity. There are some older 8GB DIMMs that are dual-rank from when 4 GB DIMMs were more prevalent.
https://en.wikipedia.org/wiki/Memory_rank
Anyway, back to the Zen IMC. The Zen IMC is set up "weird" so that it can sort-of benefit performance-wise from using MORE ranks, so that someone using, let's say, 2x16GB DIMMs can (sometimes) eek out as much performance with DDR4-2666 dual-rank versus someone using DDR4-3200 in a 2x8GB configuration. Sometimes! But not all the time. #1 problem on the DDR4-2666 setup is lower IF speeds, meaning higher interCCX latency. Boo. So generally-speaking, you want to populate each memory channel with exactly one rank of DRAM, and that means 2x8GB giving the highest possible DDR4 speeds.
So for example, if you spend a lot of time hand-tuning your RAM, you can get DDR4-3466 or even DDR4-3600 on a 2700x in a 2x8GB configuration using Samsung b-die. If you go for 2x16GB, about the fastest you'll get is DDR4-3200, and that's only if you buy some of the fastest 16GB DIMMs out there - the ones advertised as DDR4-3200 (or higher) AND are confirmed to have Samsung b-die on there.
If you are going to populate all four RAM slots with 16GB DIMMs, you are now running 8 ranks of DRAM off your IMC, which will give it fits. I would not expect to b e able to stabilize RAM at any speed above DDR4-2400 in that configuration. Actual performance? Unknown, I've not seen it benched. I've only seen 2x8GB vs 2x16GB comparisons with some "interesting" results. The lower memory speeds on the 2x16GB configurations aren't ALL that bad. Not as bad as you'd think. But it takes a lot more work to get it functioning.
The worst thing is that most RAM kits have XMP settings configured for Intel, NOT AMD. So on desktop systems, a lot of the time, what happens is you buy some RAM and set everything to default, and it runs at a. . . DDR4-2133. For example, my 1800x on an x370 Taichi + DDR4-3733 DIMMs defaults to DDR4-2133. None of the XMP profiles on my DRAM work, at all. I have to hand-tune everything, no matter what.
FlareX DIMMs have some settings that are *supposed* to work okay on AMD setups. Not sure if there are any 16GB FlareX DIMMs. But in a 4x16GB configuration, I'll bet those settings won't work anyway. So, hand-tuning for DDR4-2400 or maybe DDR4-2666 would be necessary.