UKCats;
Plate material furnished under the ASME SA 515 specification is for use in components or vessels that operate above ambient temperature service because of the coarse grained structure of the material, resulting in lower impact or notch toughness properies. So yes, SA 515 plate material has poorer impact properties in comparison to SA 516 plate material, which is supplied as fine grained material. I believe the SA 515 pressure vessel plate specification replaced the original SA 285 plate specification that was used in many ASME Section I boiler components like waterwall drums, etc.
For vessels or components that operate above ambient temperature service, SA 515 is acceptable plate material because you would be above the ductile to brittle transition temperature in service. For vessels that operate at or below ambient temperature service, the SA 516 plate material would be specified because of improved impact properties or notch toughness per ASME Section VIII Div 1 or Div 2 mimimum design metal temperature requirements.