they 2.25 inches so bigger bore than standard and it is straight through so u might get 1 / 2 bhp but lose a bit of torque due to increased bore
from what i have been told it is better to have bigger bore manifold / downpipe / cat then reduce size for centre section backbox so healthy increase in bhp and torque
if u got a standard irmscher backbox on there then it is designed for under 2 inch flange, it will fit fine but u might lose a bit of flow when fitting a racing tube
that is why the likes of johnnyn goes for a straight 2.5" cat back
exhausts uk are doing 2.5" cobra cat backs for under 200 quid
also road runner do cheap magnex cat backs for the vectra
personally i have
piper tubular manifold
custom 2.5" downpipe and 200 cell sports cat
magnex 2.5" cat back
and she sounds lovely when warmed up