I have an alternative suggestion for Markerlight outlined
hereThe idea is that most main Tau weapon systems benefit from Markerlight, which improves the accuracy of the weapon at extreme ranges. To achieve this, the Tau make extensive use of Drone and Sentries sent forward in advance of the troops to identify and 'mark' targets.
Tau weapons that are linked to "Markerlight" systems have the accurate range of the relevant weapon increased by 50% and +2 added to the to-hit dice.
But to balance this out, obviously the normal stats would need to be reduced accordingly to those of equivalent IG or Eldar weapons:- 30cms, 40cms or 50cms maximum, while the to-hit values would be reduced by (2).
so for example the revised weapon stats for a HammerHead might be something like :-
Weapon . . . . . . . Range . . . .Firepower . . . . . Notes . . . .Railgun . . . . . . .50cms . . . . AP6+/AT5+ . . . . . Markerlight Guided
 OR
Ion Cannon . . . 40cms . . . . AP6+/AT6+ . . . . . Markerlight Guided
Smart Missile . . 20cms . . . . AP6+ . . . . . . . . . Markerlight Guided, Ignore cover
Seeker Missile . . 60cms . . . . AT6+ . . . . . . . . . Markerlight Guided