A throttle body spacer will create more plenum volume, a manifold spacer (for our Honda's) will create added runner length. I can see added runner length creating more potential benefit than the TB spacers, but it depends on thickness and material used.
I can't see the spacer being anymore than an inch thick, otherwise you're gonna run into potential clearance issues with the firewall. I'm not sure who the manufacturer of this spacer is, but generally they are cheap to make and easy to sell. There is no R&D going into most generic spacers, basically they machine it based on port size/shape and that's all. I could be wrong though...
I see no reason for added plenum volume beyond the Y8 manifold though (besides boosted applications or balls out racing machines), it's quite large considering the engine size. You could get more benefit from porting the manifold itself. The ported Y8 manifold i had on my Y7 engine made a very noticeable difference in all RPM's, even though mine was an 4sp auto w/250,000mi on the odo. I'm not sure how much of a benefit it would have over a stock Y8 manifold though, but it fed my engine well.