I never considered the mileage of the 8.1L (6-8 mpg) in my 22,000 lb. 37' ex-gasser to be bad. Of course, I was comparing it to the mileage of the 5000 lb. Dodge 1500 Hemi pickup we had at the time which got 8-9 mpg empty.
Couldn't get rid of that Hemi quick enough, but it did make me appreciate the mileage of my motorhome.
I too think the spacer would do nothing but take money out of your pocket. If they were such a benefit the OEM's would jump on them- it would give them an advantage over the competition.