Mmm, Yeah I fell for this early on in the game...
Any brush that is assigned a texture
regardless of the type of texture will be rendered by the engine. If the engine has to render the texture on a brush
even if it won't be visible in the game then the poly count will go up. AAATRIGGER is only "unseen" when it is applied to an entity. So if you apply it to a normal brush, then it will be rendered and you will see it.
Confused yet?
Entities do not increase r_speeds, models and textured brushes do ! So there are no savings by using the AAATRIGGER texture on an entity, it is there to make it easier to find your triggers and buttons in Hammer.
There are some savings by using the SKY texture on brushes that can not be seen by any player, but I have yet to prove the theory, and the Null texture works well also, but the place to use this is on roof's and unaccessible rooms.... Caution though, you can stuff this up, as I did last night :
My advice for what it is worth, is to map like there is no tomorrow, compile often, and check the speed. If it starts to go over 600, then look at the areas of possible saving. For a unique insight look at the tutorial called
R_speeds