1 meter per vertex v.s 2 MpV

We have an open-world game currently in development and looking to take our map from
8x8 km to 16x16 km
Currently we are using a Houdini to Unreal Heightfield workflow. Due to memory loads, I am unable to bring in a 16x16. I was contemplating X-Y scaling the 8x8 200%. Doing this would bring the landscape to 2 MpV.

What type of issues would this bring on?
anything that would be game-breaking?

A lot.
Memory wise it likely destroys performance at the occlusion level.

Lighting wise the shading errors would be much more visible.

Instead of doing that, since there is really no limit to the size of anything if you do things right, find what size of 4 levels (landscape or mesh) you can keep comfortably loaded in your target machine for your heaviest game scenes.

Once you have that, just cut up the project in tiles of that size and set up a system to load/unload them as needed - be it world composition or custom.

Thank you. That is what I expected.
Looking at the 5.3 roadmap it seems that they are trying to build in the functionality that
I would need. Have a lot of other things I can work on while they implement it, if not I agree that a custom solution will most likely be the way to go.
Thank you again for your time.

There should be no issues with importing a 16321x16321 heightmap into a World Partition level if the computer has at least 64GB of memory.
Heightmap importing doesn’t seem to like to use larger Page File setups.

You could use World Composition, which allows for subsequent tile importing, but that has been deprecated in UE5.x.
World Partition can also be extended to larger sizes in 5.0 to 5.2 by using the tile brush import system (I can provide a YT video link if you are interested).
For UE5.3 World Partition subsequent heightmap importing, I plan on testing that feature shortly to see how it works.

UE5.3 Preview seems to only have a new Landscape Cell “Region” for larger selection of a region to load and unload.
There doesn’t seem to be any new “subsequent import” feature yet that I can find.

I very much doubt you can import a file that’s double the maximum engine allowed file size without having modified the source to allow larger files…

World Partition Landscapes larger than the current UE5.1/5.2 maximum of 16384x16384 can be done using the Landscape Add brush import system.
I have a video on my YouTube channel showing exactly how to do that.
Note that UE5.1/5.2 error with a Max2DTexture error when attempting to import a single PNG-16 larger than 16384x16384.
But you can get around this using the Landscape Add brush import feature.
There are developers I know of who have created open worlds up to 90km x 90km at 1-meter using the Add brush system. It is just a lot of work to do.
UE5.3 Preview doesn’t appear to have the subsequent import feature in it listed on the Roadmap, so Add brush is currently the only way to create larger World Partition worlds.

Just use the older system. Import the correct size tyle set for perfomance.
That’s all.
Anything else is a pure and total waste of time.