I asked this question as well a while ago and no one gave me an answer. I assume it adds a shiny part to the intake and that is about it.
Pete, please see attachment. Many smart people have researched this phenomenon, see biblography, attached.
Regarding the spacer thickness, there's an interesting photo on page 48 of Corky Bell's book "Maximum Boost" that shows a plenum spacer on a Esprit that's about 3"+.
That spacer is huuuge! I'd say, it is thicker than the cover (3.5" OD), so it must be 4" thick. Book does not mention the spacer at all. This makes me think that Corky used someone's else picture. I'm wondering whose car it might have been? It was a long time ago, because this book was first printed in 1997.
85-88 cars are limited to 1-1.5" spacer thickness, because of the plenum cover proximity to the air filter box. Additional 0.5-0.75" of clearance can be gained by grinding off 3/4" of the flange (@ top of the "beak") of the air filter cover.
89+ cars have much more room available.
Personally, I'd like to go bigger.
I'm wondering if you would make me 2" thick part. CNC water jet program operates in x-y plane. There is no Z component, so thickens of the plate should not make any difference.