I recently came across this special 4-hole carb spacer that extends into the plenum of a single plane intake manifold. The company that invented the spacer says it's good for several hp (15-35).
Anyone tried this spacer?

Looks a lot like an HVH super-sucker.
Yes, they can work very good. Just bought one for my 360 to try because it has a single plane on it and this should build a little more midrange hopefully.

HVH, Wilson and several others make slightly differential variations of that spacer. They help signal strength through the venturi and booster. Most times works better than just a 4 hole. Depends on how much loss there is from your carb/manifold combination. Some will gain more than others.

This one is different from the other tapered spacers as the center piece extends an inch into the plenum. No other company has a spacer like this as Tooltech has patented this design (patent US 6595175 B1).

Yes, I use this spacer. The inventor of this spacer is no longer with us as he passed away in May of this year. His name is Thomas "Corky" Capoferi Sr. of Corky's Competition fame. I was one of the first racers to use this spacer in the late 70's and it was developed over time into it's current form. The reason it looks like this now it that you can use single hole spacers to increase the plenum area for fine tuning of the air fuel flow at the bottom of the manifold preventing fuel separation in the air stream.

AED has their Vortex spacer that also extends into the plenum, but it's shaped a bit differently.

There has been various variations of that spacer type made by several people.
The trick with carb spacers is results will vary depending on your application. I've made several for myself, but am reluctant to sell them for that reason.
Something that might make big gains on a 300 ci high RPM engine, may not do much on a 300 HP 360 with a dual plane. They're not a one size fits all kind of deal in my experience.

A carb spacer, no matter what design, is one of those things you just have to try. Each motor and manifold differs so much that you simply need to try it and see if it works. Most of these spacers, like the one being discussed here, are so similar that I doubt you'd see much difference at all b/t a pile of them from different manufacturers.