The Horizon Walker prestige class special ability Terrain Dominance includes the following clause/sub-ability: "When dealing with creatures native to that terrain, the horizon walker treats his favored terrain bonus for that terrain as a favored enemy bonus (as the ranger class feature) against those creatures"
So in theory a level 10 horizon walker could pick the same favores terrain to upgrade every time he gains an additional favored terrain and end up with a single terrain where he has +14 favored terrain bonus, then done a pair of Boots of Friendly Terrain to increase this bonus to +16 and thus gain +16 to all attacks and damage rolls against all creatures with that terrain type listed under the "environment" heading, right?
He needs to be a ranger for the boots but yes.