I can not doubt any Alfa's figures as many will say that "Alfa Massaged the actual figures"
The real world truth is that the engine's output all varied greatly from one to another....
This can be explained with two identical 156's 2.0 TS's as demo's, same age, similar mileage, one used no oil at all, the other used about half a litre per 1000 miles. Both driven like they were stolen and one felt quicker than the other????
It's almost as if the engines were hand built with the same tolerances and varied foibles that come with hand built engines.
As regards to your GT feeling slower than a Subaru.... Of course it will... 4WD vs FWD for a start so traction is a huge factor... Then there is also the "boost effect", the feeling of sudden torque as apposed to a more progressive wave of torque.
My old 2.4JTD felt a lot quicker than my old 2.0TS yet the Twin spark would eat the JTD for breakfast in the figures... Pay more attention to your speedo needle rapidly climbing and set targets against land marks, then you will see that it will be a lot quicker in the real world.
If your power figures are correct mate... Get a Q2 LSD before its too late... Or you'll need a new gearbox and diff soon enough