Simple problem. I just got a '90 GTU that has sat forever (around 2 years) and the car fired right up and I've got some good gas in it now. So now when I take the beast up to 7k rpm it stops pulling after that. And after I downshuft it lets out a big puff of grey smoke. Still has the stock airbox and exhuast. I'm guessing the cats are clogged. The car doesn't smoke any other time. Just after I downshift at high rpm.

 

The oil control rings are about worn out. It's not an urgent issue. Maybe port it when you you rebuild it next year.

In the meantime:
The 7K nose over - A header with a free flowing exhaust will fix that.
A free flowing cold air intake will help also.
Then your port job be amplified.