Well, I got my car back together and everything went pretty much as planned. The turbo does reach full boost now (about 18-24 psi as measured by my VDO boost gauge, known to read about 4 psi high). The first day the car was a little peppier, nothing too noticeable. But after a couple of day, there was a very noticeable difference! I now remember that the air/fuel mapping is adaptive. Since the turbo probably has been having issues for at least a year now, the ECU probably remapped in order to compensate for the lack of boost (in an attempt to maximize power and minimize smoke, with what is available). Over the course of 3-4 days, I noticed the car became really zippy again, and seems to smoke a lot less in general (don't worry, it still has the "anti-tailgating" feature if you stand on it for a few seconds).
So in the end, a 50 cent retaining clip was all that was needed, but because of what it took to get to it, along with other issues I noticed while I was in there, I probably spent close 8 hours working on it. However, it's worth mentioning that I also got around to doing some important things on my list. Most importantly, I finally replaced the glow plugs and harness, which (probably the harness) has/have been throwing an engine code for literally years now. Unless I've forgotten, I'm pretty sure the glow plugs were the original ones the car came with. After I changed them, I cleared the MIL code, and the light has stayed off for weeks now. Easily the first time in 3-4 years.
Hopefully the car will have an easier time starting this winter. Can't wait to find out...
Okay, I take that back.