How to optimize when MUST use 'highp float' on mobile using GL-ES?

how to optimize when MUST use ‘highp float’ ?
eg.

highp vec3 Func(highp vec3 t) { return t * t * t * (t * (t * 6.0 - 15.0) + 10.0); }
    highp float B =
        mix( mix( mix(x1, x2, T.x), mix(x3, x4, A.x), T.y ), 
             mix( mix(x5, x6, T.x), mix(x7, x8, A.x), T.y ),
             T.z );