Sounds to me like you have 2 distinct things happening. First, it sounds like you're seeing too little vacuum at the modulator (hard shift and thunking downshift) for a given throttle position. My car is missing almost all the vacuum stuff, and I've driven it both with full vacuum and zero vacuum going to the modulator. On the 2-3 shift I'm inclined to say you'll have trouble adjusting it out based purely on the fact that I haven't been able to on my '87 sedan yet. I have tried several adjustments of the Bowden cable, but cannot fix a quick 2-3. Right now if you hold the accelerator to the floor it shifts at 4700 each time. But, at low to moderate acceleration rates the 1-2 is good, then immediately after comes the 3-4 and it's rather annoying.

I've been in sponge mode reading every little bit I can find on here, so I'm suffering a little from info overload. But, seems like I read that there is an upgrade kit for these cars to fix the stacking? Required going into the valve body, but wasn't horrible to do... Have to go search some for that one.