The new Snap AF mode with customizable distances and new full-press mode are great and make a big difference. For me these are the best new featues of the GRD III and, even more than the faster lens or better IQ, worth the upgrade alone.
One warning though if you get the new GRD III or look at RAW samples, do no use it with ACR as the quality is not very good and the noise appears blotchy instead of fine. Best for now is to use RAW Therapee. This picture is edited with ACR and if you compare it with the ones from yesterday, which have been developed using RT, you can see that it does not look as good.