Tracy,
spacers definitely helped the older carb'ed and aspirated vehicles.
as the fuel came from the jets and was mixed in the carb with the air at the throttle plates.
EFI is slightly different as the injectors are directly in the combustion chamber. there are companies making TBI/EFI spacers for newer applications. I think one is named Vortex? or Helix? I think they make one for the GM 4.3L, but I have not heard positive or negative feedback.
if you are reluctant to spend the $70-100.USDollars, and want some personal testing experience prior to purchase you can:
go to a parts store and purchase the throttle body gasket for your vehicle. you now have a template.
pick-up a 3/4" piece of scrap plywood, or 1" firring strip of wood, and draw out your new TBI mounting surface, cut it out with jigsaw and/or dremel; sand the inside good (don't want wood chips sucked into the intake), and then get some longer metric mounting bolts for your wood spacer and TBI unit.
not as nice or as CNC milled as the billet aluminum one, but WAY LESS expensive, and you can now make a more educated decision on if it's worth it.
hope this helps,
chris
nrt racing