Nice that you tested with the DRAM calculator.
AMD needs to get on the ball with memory vendors to get an XMP-like standard onto SPDs. Ever since 1usmus' first version of the DRAM calculator I've realised how terrible Inlel-optimised XMP settings are for Ryzen. Given that Zen1 and Zen+ are mostly happy to run 1:1 with FCLK at 1600 and Zen2 being all but guaranteed to hit 1800, you'd think there'd be a big market for DDR4-3200 kits with AMD-specific timings in their SPDs, right?
Oh, right. That reminds me - I was going to tighten up the timings on this HTPC - just picked up a 32GB clearance deal 2666 kit to replace the 16GB 3200 B-die and they're still running at XMP. Silly me!
I would have liked to see how much those RAM OC settings matter in games on the AMD and Intel systems.
Intel, not much. Every core in an Intel consumer CPU uses a monolithic cluster of cores and cache. Whilst an oversimplification, the speeds of those cores, caches, and interconnects between them is fixed, regardless of the memory clock - every core sees every core's cache directly. There aren't too many applications that are bottlenecked by faster dual-channel DDR4 so outside of synthetic tests, faster RAM isn't that useful for most consumer workloads. It's been a couple or so since I read up on it, but for Kaby Lake-era I believe DDR4-2666 was the budget sweet spot and anything beyond DDR4-3200 was almost pointless for Intel because of diminishing returns.
Ryzen is far more memory dependent because infinity fabric (IF) clocks (called FCLK) are loosely locked to the RAM clocks. In a 3900X, for example, there are two CCDs (8-core chiplets), each with two CCXs (quad-core clusters) and each of the three active cores in a CCX has its own L2 cache. Despite being on the same chiplet, even the two CCXs in a CCD don't talk to each other, they talk via the IF bus which relays data between CCXs L2 cache and also acts as the bus for all the cores to see the L3 cache in a CCD.
Up until 1800MHz, the FCLK is locked in a 1:1 ratio with the RAM clock, so when you put DDR4-2133 in your Ryzen board, you are running the CPU's inter-core cache bus (among other things) at 1066MHz. If you plop a DDR4-3600 kit in there, the crucial inter-core cache bus gets a huge boost to 1800MHz. At DDR4-3733, there's an automatic divider that drops the RAM clock and FCLK ratio to 1:2, resulting in a 933MHz FCLK which is
awful. You need to jump to about DDR4-4400 before the RAM gains offset the penalty of the 2:1 divider. Either way, running FCLK as high as you can is really important for Ryzen because it's reducing cache latencies, and a HUGE cache is a big part of why Zen2 is so much better than Zen1 so any kind of cache upgrades obviously make a difference to Ryzen performance. It may not show up in average framerates, but cache misses are closely tied to your minimum framerates. More, faster cache = better minimum FPS and as a gamer I really appreciate that.
Why 1800MHz FCLK? Because that's what AMD reckon you can safely push the IF clock to. Some motherboards let you set a manual ratio higher, but on the boards I've tried it with, I haven't had RAM fast enough to test beyond 3600. I've also seen an R5 3600 that was only borderline stable at 1800 FCLK and dropped the RAM down to 3533 to prevent occasional cold-boot issues - so 1800 is likely but not 100% guaranteed, the "guaranteed" official spec is 1600 FCLK to go with DDR4-3200.
If you want to see what sort of gains you can get on Ryzen with faster RAM, just google for videos with titles like "Ryzen memory speeds tested in x games" and you'll see some games really benefit hugely from the faster FCLK whilst others DGAF. The next part is just my own opinion, but I feel that Zen 2's default PBO is good enough to clock those chips close to their limits regardless of your motherboard or cooler. Sure, if you spend lots of money on a premium motherboard with overkill VRMs and back that up with a $100+ AIO you will also get clocks up by maybe 300MHz but you are better off getting your RAM up to 3600 first; The difference between affordable 16GB DDR4-3000 kits and more premium DDR4-3600 kits is about $60. That's the best $60 you'll spend on a Ryzen - much more performance/$ than cooling or motherboard upgrades.
If you can afford it, do everything - get the DDR4-3600 and get a 280mm AIO to cool your all-core or PBO+ overclock, and get a premium motherboard to feed the overclock with beefy VRMs. But if you have the budget for only only one or two of those things, make sure you get 3600MHz RAM first. It's the
lowest hanging fruit for a lot of consumer applications like gaming, encoding, and those 150 background processes your Windows installation is currently running in the background behind your 100+ Chrome tabs
If you want a better explanation of IF dividers and FCLK, look up buildzoid's channel on YT - That's who I learned from....