You did not say if you have done a compression check, this with the vaccum gauge reading would tell you alot aboout the problem, 15 is low, and suggest late valve timming, chances are with plugged cat you would have the low power but at idle, the vaccum readings would be normal 18-20.
Unless the cat was totally plugged in which caes you would be most likely getting a backfire through the intake on exceleration.
To compare my 86' 300E 3.0 Ltr. gets 17-18 at idle slow acceleration gets up to 22, quick but not stab acceleration gets up around 25-26 then drops to about 10, then quickly return to 18.
Check the compression on all cyl.