I run a spacer but only for it's insulating properties, its a moroso 1/2 laminated wood spacer, keeps my carb cool as a kitten.
Back to your question, every motor is different some respond to spacers some don't.
Generally speaking the taller the spacers like a 2 inch open one add plenum volume which should net you more top end HP, the 4 hole spacers add torque, how much is debatable.
Some companys claim some pretty substaintial gains but every combo is different - what worked on their test mule may not perform well on your set-up, kinda a crap shoot if you ask me
JMO>Cp