Nothing related to the exhaust will be vacuum, EGR maybe, but that is mostly smog related.

When you put 85/86 manifolds on an early car you need to make about a 2" adapter on one side to correct the offset between exhaust port locations on the different model years.

Magic of the 928 is that sudden performance differences may be totally unrelated to recent changes, so best to assume your issue isn't exhaust related.

One thing I would ask myself before making and mod's is, how is the car to be used? Street driven most of the time or race car? Headers usually don't do much at normal street speeds. You might run into other problems as well. Just my input. Your money and time, do as you like. Remember these cars are old, not going to keep up with all the new ones today. I like my car just for it's looks and handling. You want fast? Go to Chevy, plop down $120K and get a sky rocket of a car in a Z07 Vette. Then take driving lesson with car so you don't kill yourself!