That's probably accurate - 2 to 3 HP on an N/A motor. But you likely won't get that on yours (and you may actually lose power) as you already have the C&L elbow, which, essentially, already has a built in spacer. Check out this this link for more info about how the C&L elbow was engineered. I wouldn't mess with what you already have. http://www.cnlperformance.com/plenum03.html
Its about the same as a carb spacer in the old days. The different mods you put on your car will effect it in a different way I imagine. A stock carb engine might only pick up 1 or no horse but a modified one could pick up as much as 10 hp from a spacer.