If you want speed, no, you are not going to beat log(erfc(x)), at least not without writing custom code, preferably in some compiled language.
If you want higher accuracy for large positive x, then yes you could get more accuracy in the upper tail. It won't be incredibly fast though. I'd probably start with the simple asymptotic expansion given on the wikipedia page.
Be careful though, as the classic asymptotic expansion actually diverges if you take too many terms. But for moderately small x, you would need perhaps more terms to get an adequate number of digits. It depends on how many digits of accuracy you need/want. The point is, you need to do some careful work to choose an expansion that will be fast AND highly accurate. The problem will be for intermediate values of x, perhaps on the order of 1 to 2.
