Blur, alpha only format, and pay with contrast to get more range. I'll sometimes do my alpha textures in 16bit so I can muck around with the curves with out losing precision in the final 8bit alpha.
It might also be worth using a sharpened alpha blend instead of alphatest if you don't need the ZWrite. Or even alpha to coverage (though I don't think UE4 has added support for that still).
(alpha - _cutoff) / max(0.0001, fwidth(alpha)) + 0.5