depends on your current ratio (5th and final). here is an article on the hows:
5th gear swap modification- lower gearing on a VW 5 speed transmission | VW TDI forum, Audi, Porsche, and Chevy Cruze diesel forum
There are gains to be had for sure, but will need more technique than cruise control to take full advantage of them.
Also the gains are manifest when combined with aerodynamic mods.