Err... I wouldn't advise the HL prefabs anyway, for several reasons.
1. They get so boring, because your by far not the only one that uses them. To me (personally) it makes a lazy impression.
2. They aren't specific made for your level, your environment, and may not 'fit' in the theme so well.
As for using prop models instead of brushes for detail, yes, it's advisable for smaller objects or natural things (trees and so). However, due to the different rendering of models (they don't use lightmaps like the world geometry does), it looks quite ugly when you use it as large scenery. It doesn't light up like normal walls too when you turn your flashlight on.
But, because of this different rendering, model polygons seem to take about 12 times less calculation time than world polygons. I don't know if that's a factor 12 but at least it costs less time.