As long as you have a use for the miles, a ratio of 1.875:1 is probably as good as you'll get. And for sure if I had like over 500k SPG I wouldn't hesitate to shoot over 100k. But I would not shoot over my entire account unless I had specific purpose.
Long term there has to be strong chance US Air will fold or at least leave Star Alliance. Without *A I wouldn't transfer at a 10:1 rate