The idea about the lower elevation water being warmer is not necessarily true. It can get pretty uniformly cold between the low and mid elevations in the park. However, in the mid elevations, you can find where the stream has more gradient, and therefore faster flowing water, and the fish out in the faster water are going to be more like to be active and looking for food, while the fish in deep slow runs (as is common in the lower elevations) are going to be hunkered down in the bottom of the pools, using very little energy and you just about have to hit them in the face to get a strike, if even then. The 1-2 degree difference is water temp is probably of minimal concern compared to having faster moving runs...
Grey Neversink produced REALLY well for me for a dry fly in 40 degree and above water last year.