Screenshots of what you're doing.

As I said it shouldn´t be done if it can be avoided. But try to clip such a shaped mesh with normal brushes by hand! Good luck!

It doesn´t destroy the performance as long as you keep the amount of triangles low and the model itself "small". At least I don´t recognise any fps drops, it´s always at about 200-300 fps (HIGH effects settings), and there isn´t ANY VIS optimisation nor is it hiding any sections of map.

Compared to the terrain parts of the current Xonotic maps which use patch meshes (!), this method is way more performance friendly. Later on, if some people still got problems with it I´ll add LOD for these parts.