I'm a big fan of Viaduct's optimization. I'm on my phone or I'd double check but I'm pretty sure it has areaportals in the gates building. The rest of the map is basically a func_detail so areaportals aren't usable.
If you do emulate viaduct's optimization, one recommendation I have: make additional skybox brushes that "seal" the slits you create, and put these extra brushes in their own visgroup.
When you need a faster compile, enable the visgroup. Otherwise disable them.
Important because cutting those holes will seriously crank up your vis calculation times, and if you're working on lighting or something where you don't need to see long distance stuff, it'll really speed things up.