I just tested 150 eld-x with Staball 6.5 in a 280ai last weekend. 25" barrel. I used the starting load and went up 0.2gr increments looking for a flat spot. Velocity was all over the place until I got 0.4gr from max then I had 3 charge weights within about 20fps.
57.6 - 3039
57.8 - 3048
58 - 3026
Not even a hint of pressure but not going any higher. Accuracy was pretty good even with the different charge weights. From 54-58 POI was about the same. I loaded up a bunch at 57.8 to test seating depth. If the ES/SD stay pretty good, will use this load.