[Ace]

This is an excerpt from my old addon that enabled it.
I don't know if it is still working. It only ever worked in a given direction, and never during combat.

Code:

if ( gMoveHack == "SpeedHack" and not Player.incombat and not MeshManager:IsKeyPressed(016)) then
            if ( MeshManager:IsKeyPressed(017) ) then
                if ( Player.ismounted and currentSpeed ~= 18 ) then
                    Player:SetSpeed(FFXIV.MOVEMENT.FORWARD, 18)
                    currentSpeed = 15
                elseif ( not Player.ismounted and currentSpeed ~= 12) then
                    Player:SetSpeed(FFXIV.MOVEMENT.FORWARD, 12)
                    currentSpeed = 12
                end
            else
                if ( Player.ismounted and currentSpeed ~= 9) then
                    Player:SetSpeed(FFXIV.MOVEMENT.FORWARD, 9)
                    currentSpeed = 9
                elseif ( not Player.ismounted and currentSpeed ~= 6 ) then
                    Player:SetSpeed(FFXIV.MOVEMENT.FORWARD, 6)
                    currentSpeed = 6
                end
            end
        end