Update this morning:
Performed compression test using Harbor Freight Maddox Compression Tester (questionable quality??)
Dry Results:
Cyl. 1: 120psi
Cyl. 2: 120psi
Cyl. 3: 150psi
Cyl. 3: 150psi
Wet Results were kind of all over the place:
Cyl. 1: 140psi
Cyl. 2: 130psi
Cyl. 3: 160psi
Cyl. 3: 160psi
I also performed a crude leakdown test @ ~50 psi from my air compressor through the compression tester attachments. For all cylinders at TDC, pressure built in the crankcase. When I loosened the oil fill cap, there was enough pressure in the crankcase to lift the fill cap. Not looking good for piston rings and/or cylinder bore ☹️

 

1 and 2 are less than desirable. It will probably run for quite a while still, if you keep the oil level full.

The problem is that, just like with the tester, continued operation will pressurize the crankcase, pushing the oil vapor into the intake manifold, messing with the AF ratio and fouling the intake valves. This is when not boosting. When boosting, it also sends the excess oil vapor into the turbocharger, charge piping, intercooler, then to the intake valves, etc. May also have a bad effect on the catalytic converter.

Catch cans would be a good stopgap measure.

 

Thank you for the advice thus far. Unfortunately, this makes a lot of sense. About a year ago, I started getting a recurring P2188 code almost every time I would drive the car to/from work, it would idle a little rough every now and then, and I was getting random misfires when I would floor it to pass someone on a 2-lane highway. I thought it may have been related to the High Pressure Fuel Pump or the PCV system. I replaced the HPFP; same symptoms. However, if I disconnected the PCV line between the intake and top of the crankcase, all of my issues would go away except for the misfires. I'm sure the vapors have been getting past the rings for some time now, and now it is bad enough to start pulling oil past the rings.

Now to make a decision: keep and fix or sell for cheap to someone else as a project...

 

How's the condition of the rest of the car? Zero road salt exposure? Some of these SS's are worth a new engine.

 

Honestly, the car is in pretty good shape. The body is probably a 7/10, and the interior is probably an 8/10. Some undercarriage rust, but nothing structurally concerning at this time. I have stored it inside for the past 3 years I have owned it. I've thought a lot about it this week, and I am likely going to keep it and rebuild the engine with pistons, rings, bearings, turbo, etc. for $2k from ZZP or replace the engine with an LDK from ZZP for $4k. I don't have that kind of cash just lying around at the moment, so it will probably get parked in my pole barn for the winter or at least until after the holidays! It doesn't look like the engine will be terribly difficult to pull once all of the hoses and cabling are pulled off.

 

You need a proper leakdown for a better guess as to whether it's the cylinders or valves that are leaking.

If oil changes were deferred or the car sat, sticky rings are possible. You could try a tablespoon of Seafoam or ATF in cylinders 1 & 2. Can't hurt.

I like your $2000 rebuild. You could start by pulling the head, and might discover you need go no further. Cleaning up the (coked?) valves (if not bent) and making sure the guides seal may be all you need. Pulling the head and inspecting is free if you do it yersef.

A turbo motor remains pretty driveable even with poor cylider sealing, because... its got a turbo. But of course, the excess pressure in the crankcase pushes oil mist to the cat valves, accellerating coking, not to mention blowing seals... unless you install a catch can, which is a great idea for any well-broken-in DI turbo engine. ZZP sells a filter type filler cap, which can be messy but will ensure against excessive crankcase pressure. A light foot on the throttle helps too.

GM sells a kit for decoking DI valves. Pretty $pecial. CLR has a cheaper simpler kit that I've used. Helpful to read the GM directions even if you use the CLR kit. The decoking procedure prolly won't fix heavy coking, but in such a case it might clean up the stems enough to forestall sticking and sealing problems.