A mate of mine has a standard G reg Astra GTE running a proven 158BHP. My CDi also runs the same spec but I don't have a RR repport for the BHP..
From the off mine pulls alot more than his right up to the 100MPH 'test' make. I've driven his car to see if its driver error but we get the same result. Also from rolling speeds of 30 - 50 - 70 MPH I pull easily ahead.
I know some XE's are better than others but his runs really smooth and is running good power figures etc. Bit cars weigh roughly the same with the ICE etc thats installed.
Also have the same wheels/tyres etc. So the cars are pretty much even in most ways.
I know mine isnt quicker than it should be so what other reasons could there be for such a difference in performance..