All I say is, the e_poly limit is something you have to dynamically determine for yourself, depending on the situation. The engine stays the same, a mod however might have it's specific situations.
Say, a mod uses ragdoll physics (yes, this is being done in a HL mod). I can imagine this can be very computionally expensive. So, it also affects fps, in wich case the mapper should go with less w_poly's and e_poly's to keep the same performance. Unless they're aiming for higher-end computers, in wich case they could go with higher limits.
And when you have a lot of w_poly's, you should have less e_poly's in that area, where a place low on w_poly's allows much more e_poly's.
It's just very dependant on the situation and the aim of the mapper.
And that's a very good solution, jaardsi. I wouldn't have thought about that myself...