I would say the 15-20 is not going to happen. I have no dyno numbers, buttttt.. My butt dyno felt a real improvement when I put the Headers, Exhaust, and ram air on. Also saw a little raise in my trap speed with these.
Then I put the ECM on, it was later then the rest of the parts, I felt no improvement and didn't notice any difference at the track. It actaully seems to go a little flat compared to the stock ecm at 5500 or so. But this could all be in my head, I have been meaning to put the stocker back on but haven't got around to it yet, just to see if their is a difference.