A Land Far Away

Is it possible to turn elevated walkable placeables into environmental objects AFTER the bake has been done, and still be able to have a functioning walkmesh?

Similarly with walkmesh helpers? Do they need to be left in place after a bake, or can they be removed?

Would this have a significant effect on area performance on a server?

You can change the Appearance of a walkmesh placeable to [None] and retain the collision data and walkmesh - this is how the Moonbridge is done.

Changing a placeable to an environmental strips the collision data from the object.

Walkmesh helpers, specifically, are deleted by the ACR when your module loads. The performance impact of doing so only happens once, and overlaps with other area-based checks we do, but the saved toolset time of maintaining areas with walkmesh helpers by just leaving them in and letting the module's startup event clean them up is significant.

So, yeah, leave the helpers. Other placeable gymnastics require more fiddling, as we can't reliably identify any other kind of placeable as existing just to change the bake at the ACR level.