A vacuum leak will certainly kill the brake assist.

How about cam phasing, did you dail-in the camshaft?

 

OK, LT1 Edit says I'm pulling 43 Kpa at idle. If I did the math correctly, that's 12.7 inches of vacuum. Should be plenty, right?

I pulled off the hose that goes from the manifold to the brake booster after I shut the motor off, and the "swoosh" I heard indicates that there's at least some vacuum in there, although I have no idea how much. Any idea what the problem is? I'm getting absolutely no vacuum assist at all when I step on the pedal.

 

If LT1 edit is anything like AutoTap, you'll need to find out atmospheric
pressure at your elevation, then subtract the value found in LT1 edit to
get your gauge reading.

If you've already done that, then 12 in./hg. is sufficient to assist with brakes.

 

According to my weather station, the dry barometric pressure is 28.6 inches. That means I have close to 16 inches of vacuum, right?

 

Yes. You can also try getting a reading from LT1 Edit by scanning the MAP
sensor without the engine running (instead of using a weather station).