about 100 mph to about 66 mph. This is at about 7000 rpm. The larger intercoolers slow the flow in the intercoolers even more. This allows the intercoolers to extract more heat. This to me is a reasonable explanation for why there is a significant improvement in hp. In my opinion, the reduction in flow restriction is not enough to account for all the hp gains.
If gains were all due to reducing flow restriction, the larger inlet pipes would make a huge difference since the inlet pipes see a difference in volume when the boost in raised.