how effective is the throttle body spacer on the 6.0l

on an 04 f-150 with edge computer programmer and exhaust how effective in hp and mpg would a throttle body spacer help if any? i have had them on gas burner and would say its a waist of money but diesels seem to be alot different in power adders so is it a bunch of bs or legite?

Diesels don't have throttle bodies, fuel rate is what regulates engine speed, and air flow is constant, as long as the filtration is clean. 

Actually the early 6.0 had a throttle body/plate with a TPS..

I sit corrected, my apologies.