Good call changing the plugs and the filter. You might also check the AMM/Turbo/Intercooler hoses for any leaks or holes from years of wear. The fact that you can only get halfway up the yellow indicates (in my experience) either your boost is set low or your have a small leak apparent only under higher pressure. Pay particular attention to the output hose from the turbo as the low point can allow oil to pool and rot it out.

A few ounces in the intercooler is a bit excessive. Max you should get when draining the intercooler is 2 tablespoons or so. I would suggest you do all the aforementioned, drain/clean your intercooler, run for a week, and then check the intercooler again. This will tell you if the turbo is throwing lots of oil or the intercooler had never been cleaned.

As for performance, its also possible your exhaust system is in ill repair. Do youy have papers or a work history on this car? When was the cat replaced? Muffler and plumbing?

good luck,
rt