I was going to post earlier, but held off; Jeremy, I'm wondering how you're so sure about what's going on despite the lack of gauges?
My initial diagnosis was a stuck open wastegate, but you seem pretty confidant that it's not that. A BOV popping is going to ruin your party only after you've made some boost, so that kind of rules out your lack of low-end power. It would only affect higher rpm action, and I don't have to tell you that it should be deleted at the first opportunity

. You say your timing is advanced, with plenty of clack. So that rules out timing causing an issue. (just as a side note, have you tried adding wax to the fuel, as the late Hagar suggested? I have yet to verify the objective numbers, but I have reason to believe in his findings regarding dynamic pump advance. FWIW).
As far as punching the throttle goes, keep in mind, these aren't gassers. We don't get that instant snap the moment we punch the accelerator to the floor. But you already know that; so, I guess I would have to ask--what remains that hasn't been answered, and that you don't feel confident about (and/or haven't verified w/ numbers)? Is this a purely subjective "problem"? I know for a fact that my brother's diesel-swapped cabriolet feels significantly more grunty in the low end than my dad's 86 jetta (both w/ 1.6); jetta has a T3, cabriolet has a k03, and that's pretty much the only difference, aside from the weight. If anything, the jetta should be better off fuel-wise, as it's a full turbo motor, and the cabriolet only has a NA setup w/ a k03 slapped on. As you've said many times before, tiny turbos make a big difference down low; if you're used to driving that TDI swap in a mk1 shell, I'd guess that your butt dyno needs an adjustment in order to accurately gauge "normal" 1.6s
But yes, I agree that it needs better exhaust. That would be the first thing I would do for my dad's 86 jetta, if I had my 'druthers. And then an intercooler, naturally. SO DO IT ALREADY, AND POST PICS