Performance sag, I have learned, in direct proportion to the NUMBER of triangles the engine has to draw--not the size of those triangles. A rectangular brush face will consist of 2 triangles. So you're asking if it's better to have 10 tris, or only 2 tris but you won't be able to see parts of those tris. What I'd do is just keave the floor as one brush.
However, if you're using q3map2
to compile, the TJunctions process and MergeMetaTriangles process will pretty much ensure that the same thing will happen either way--I think.
In my opinion, you can be lazy here and leave it as one brush. Although performance won't really be affected one way or the other with just a few tris.