Buying an OEM PC and then having to buy an OEM key doesn't look right, at least to me.

I like how you say "only" 10-20% decrease, as if it was something negligible. That difference also translates to game performance, and a 10% difference can be a huge one, even more than what sepparates two different GPUs in some games.

Here's a couple of tests with different DDR4 modules:

http://www.legitreviews.com/ddr4-memory-scaling-amd-am4-platform-best-memory-kit-amd-ryzen-cpus_192259

testing with an X370 board, a Ryzen 7 1700 and a GTX 1080

There are 7fps between a 2400 and a 3000 kit. Not noticeable with those already huge numbers, but...

http://www.guru3d.com/articles-pages/amd-ryzen-7-memory-and-tweaking-analysis-review,1.html

Here we have 5, 4 (this time it's with high settings, not medium) and 8 fps between the 2400 and the 2933 kits. That's almost the difference between a 470 and a 480, for example!

So, I have to disagree with you. It does matter.

Yeah, it matters.
Apparently less that I guessed (though I knew I was overshooting it a bit with 10 - 20%).
It matters a fair bit but lets put it this way: is he going to over clock the system?
Unlikely the difference between 2400MHz and 3000MHz RAM is about $20 for that same amount of dollars he could get a R5 1600x and have a system that makes up the difference.

Now do I recommend that he get faster RAM if he can afford it, yeah; but if we are cutting corners, and I don't know the kind of budget I am working with I feel it is an adequate trade off.
3000
2400

R5 1600 (includes a decent cooler)
$210

16GB Vengance LPX RAM
$150

EVGA 1060 3GB
$215

Corsair Carbide 270R + 500W PSU
$100

Samsung 960 EVO 256GB
$128

WD Blue 1TB
$46

Asus Prime B350M-A
$70

Total $919 USD

You can cut back buy buying a much cheaper case and / or not buying an SSD (or get a cheaper SSD).
Cheap SSD $54
If you have no intent to ever overclock you can buy a cheaper motherboard.
If you want to play games at a higher res (1440p or maybe 4k) maybe look at getting a RX Vega 56, GTX 1070 or better.

Budget - Ryzen 1600, AM4 B350 board. 16GB 3000mhz+ RAM. GTX 1060/RX 580 and above. Will do for all modern games and be nice for productivity with 6 cores/12 threads. Will hit 60 FPS at 1080p and will do well at 1440P.

High End - i7 7700K, 1151 board, 16GB RAM. GTX 1060/RX 580 and above. Go with GTX1070 if prices aren't prohibitively expensive in your region. will run most if not all games at 1440P at 60FPS and higher, or wait for Vega 56 (AMD's GPU equivalent). i7's still best for gaming and do several tasks faster than Ryzen despite "only" being a quad core. If buying a PC for; primarily playing games, want the very best FPS and play at high frequency (over 100hz+), go Intel.

As others said, M&B2:B launch hasn't even been precisely announced yet, so you'll better wait for AMD to release more of its new CPU, GPU and APU range.
Besides a powerful CPU, to play that kind of game with the highest possible number of NPCs and other active items, you'll need enough RAM, at least 16GB should be enough for now, but leave enough free memory slot to add more RAM later, if needed. If you ever think you'll need more RAM, if you don't need it immediately, buy it later to pay it less, but make sure to buy it before it goes out of production, as from then on the price of higher quality types will only increase, only the cheapest and most mass produced modules will remain cheap for a long time before becoming rare and expensive items.

HW requirements are predicted to be not excessive, anyway http://www.game-debate.com/games/index.php?g_id=4517&game=Mount%20and%20Blade%202:%20Bannerlord , but Windows is a memory hog, and you cannot turn some utilities like antimalware off when you play, if you want to stay safe (but you can enable "game mode" on them to minimise non gaming-related data traffic when you play, and this firewall function could actually make PC performances more regular), so don't trust that 8GB requirement (let alone those 4GB minimum requirements, I have 4GB on my current 8yrs old desktop PC and they are few even for relatively old games), 16GB is a safer choice.

I wouldn't buy from Center com anyway.
I have special permanant discounts with Scorptec, uMart and AusPC Market anyway. - PC Case Gear is great for finding something more obscure or something that is out of stock elsewhere.
I have been nagging PC Case Gear to give me special discounts for years, I have spent over 20 grand to date with them... Haha.
Instead they just keep throwing little figurines in my orders.

Scorptec would get me a Ryzen 5 1600 to my door tomorrow (Half way across the country to South Australia) for just under $300 AUD. $20 is a small price to pay for speedy delivery in my opinion. Plus an Australian warranty.

It's Ryzen. - You don't even want 3000mhz Ram.

You need 3200mhz memory to get the most out of the chip.
Incase you aren't aware the Infinity Fabric/IMC is tied into the Ram's clock speed.

And please use Samsung based Ram sticks... As Ryzen plays best with those.