The carb spacer has different ways it help different combos from what I have learned. The 4 hole spacer will improve the signal to the carb, so if you are running a big cam and big plenium intake with low vacuum signal it will help in that area, which in all honesty should show gains all the way down the track. The open spacer would effectivly increase the size of your plenium so in the caes that you had a combo that would benefit from a larger plenium intake it would help, and again should show gains all the way down.
A carb spacer is something that needs experimenting with, if you have a few different ones, take them to the track and make back to back runs with the different ones and see which one shows the most gain, some builders have seen as much as 30hp gain with one on the dyno, and some have lost that much also.
I think if a man had a very well sorted combo with a trick carb, there wouldnt be much to be had, but all of us arent fortunate enough to have that.