You obviously missed the part where he said he has a T56. As in, a 6 speed transmission that has not one but TWO overdrive gears. 6th gear is either .63 or .50 on that transmission, so the 3.55's aren't hurting him at all. He can probably cruise at 70mph at 2k rpm or less. Hell I run 4.30's with my T56 and I'm doing 75mph in 6th at 2,500 rpm.
do you suppose that part of the problem could be that with the double OD and 3.55's the final ratio might actually be too low and the tune can't put enough timing in to take advantage of the lower RPM and be efficient?