I ran a 3/8" spacer on an Explorer intake. No hood clearance problems, and the upper definitely stays cooler. The spacer improved ET on a hot lap at the drag strip by 2 to 3 tenths. With the spacer, I could get two back to back runs that were very similar, without it, the second run would always be much slower.
One note though, if you did not block the EGR passages at the heads, or run non-emissions heads, the spacer won't help much because the gasses in the EGR system will heat the upper intake significantly.
-Matthew