Thank you. It is inspired by Pitch Black! Haha. Those corners aren't really that nasty, VHLT does great job on smoothing compared to previous tools. Such bugs happen when a surface/polygon is tinier than a single unit of the lightmap on it. You can fix it by lowering the scale of lightmap (by lowering the scale of texture). Anyway, GoldSrc is just too old for such level of complexity, especially when it comes to brushwork. But if you map very efficiently, I'm sure you could use it in some places.
Here, some up-close texture testing:
From left to right:
1.Original 1024x1024 to 512x512 (scaled by nearest neighbor)
2.1024x1024 to 128x128 (bilinear)
3.1024x1024 to 128x128 (nearest neighbor)