There are probably some evaluations here on the scene, but here's my personal experience -
I went up against another FX on the highway with the 1/2" spacer (I have the 5/16), but he also had a different intake and exhaust than me. I pulled ahead of him on the highway at WOT from 2nd through 4th gear. But once we got to 5th, he started catching me and pulling ahead. Deal is this - 5/16 is better for the mid range rpms and the 1/2 is better for high end rpms, which makes sense.
This was absolutely a great mod in terms of bang for the buck. But I agree it's not necessary to go with an expensive spacer when it really is just a thick freakin gasket. I think it really depends on your future mod plans. I would think going with a non-metal spacer might also give you a little thermal soak protection, sort of like the Motordyne iso-thermal version already out there.