I know that on a dyno run with a carb spacer my brother picked up 15 HP with a 1in spacer. The theory is that the fuel atomizes more and less droplets are pushed into the combustion chamber. With fuel injection, that should not be a huge issue. I agree with the above claims, that the spacer is used for elevating the height of the upper plenum, so the valve covers will clear. I can not be sure that the temperature difference would matter much as the air is flowing so fast into the engine, it surely would not have much time to change before it hits the combustion chamber. I could be wrong. I used a set of Fox valve covers on an SN95 and had to grind some of the upper plenum ears off. A spacer would have helped. I am now running a Lightning intake on a 351 with the Ford Racing valve covers. Still needed a spacer.