As for the meshes stuff, all you need to do to have flat shading is to prevent vertices to be shared between different polys (unwelding), and the end result is a mesh that looks very similar to BSP in general.
However, it does have plenty of downfalls: it doesn't cast or receive any shadows, they still don't have any lightmaps generated for them, you cannot have lower scale repeating textures unless you increase the number of polys instead, it doesn't render detail or macro textures as far as I know, the collision is at most still one of a mesh actor (a cylinder), which means that all in all, it isn't really a good BSP replacement by itself overall.
But, I use this flat-shading for effects in my weapon mods ("broken BSP" debris), and it works quite well in that context, but it also works nicely if you have a relatively high-poly shape (high BSP-wise) or a small enough shape (a few uu wide) that you want to replicate or set as decoration (such as a statue, a grid, a rail, a light, etc), or even for moving or rotating BSP (such as a fan, spike traps, etc).
Meaning that I wouldn't use it for big structural BSP things, for that BSP should always be used, but for either small details or high-poly ones, a mesh of this kind can be a good alternative and it may still blend in quite nicely with the rest of the BSP, but it will always depend on the use-case too and some trial and error to check if it does look good enough or not.
Having that said, in this particular map, the only places I would personally use this kind of meshes, would be to replace the crane ropes and claw, all the the small pipes/electricals and antennas, the rail supports in the 3 chimneys, and maybe a few other small things here and there, but nothing beyond that.
Although, if these are all set as semi-solids, it shouldn't be much of a issue overall, unless you have a LOT of them around the map, and unless you get occlusion bugs which force you to set them as solid.