It doesn't seem to help the gas mileage, but increases the power noticeably (probably why the mileage doesn't improve!
That said, I like having it over the first V-10 I had that didn't.
We go to the High Sierras a few times a year, and some of the passes are long and mean, but we get through them quite well, towing a Jeep Liberty or Wrangler.
2005 Georgetown 342DS XL, Full Banks Package
2005 Liberty Renegade 4X Toad
1989 Wrangler Sahara Toad
1984 19.5' Beachcraft i/o 5.o liter, 4-bbl (and you thought your Moho sucked gas!)
D/W Carole and Maggie the Teacup Lab (Black Chihuahua)