Putting a auto on a Dyno is always a little disappointing unless you lock the converter.....any idea what type of Dyno it was? Dynojet or mustang Dyno?
In the G8's, from the factory, the converter would be locked in 3rd and 4th in manual mode at WOT, and 4th would be locked in Sport at WOT. Most dyno operators would choose 3rd or 4th gear in manual to dyno this cars, so they don't have to do anything to lock the verter.
And those numbers are low for a car with LT's, unless the car originally dynoed 280rwhp stock. in any case, GT's are not exactly high reading dyno queens, the OP might want to take the car to the track, that would give him a more accurate representation of power. dyno numbers are just that, numbers, and some times they're simply silly. the only important reading you have from a dyno session, from the end user perspective, is the AFR graph, and that is not even included in the OP dyno sheet.