68.... Do you know if the version of v8 the other guy had was modded for the Corsa?... I believe the tune is a little different if LMS knows about the exhaust... like when they adjust for downpipes, t-stat, CAI, etc.
Still 9 dynos with three sets of DPs on my SHO when it was stock tuned showed 12HP (catted)-17HP (non-catted) increase on the dyno, but as chris pointed out... and I notice driving, the big benefit of DPs is the more rapid spooling of turbos (1500 or so rpm instead of 21-2200) giving you more torque down lower.
back to the rest of the exhaust.... I'd ask LMS about the exhaust mods....