I just updated a map with builded cubemaps and I tested it and now building have reflections. I want your opinion because I haven't played before, I always play on mat_specular 0 because I want more FPS. I only made three maps in Hammer, still learning. I hope that it will work just fine. If not, then disable reflections. I want a break now.
.. and a minor change - now map icon shows up in the map menu Garry's Mod, stupid mistake I named png file wrong...
Enjoy