Deb; The exemption you found is what my friend Walt pointed out (Which I sent you via e-mail) and why I keeping harping on the issue of "whether impacts are really required, or not." I was convinced as of our discussion last Friday, after reviewing your site specs that impacts were NOT required and remain convinced they are not in this instance.
Now then, as to ASTM vs. ASME; while the materials may, for all intent and purpose be virtually identical, those listed as "SA" materials have been approved for work under the Boiler and Pressure Vessel Codes, and MOST have been assigned P-Numbers in ASME IX. Those listed as "A" materials are ASTM's that may, or may not be used under ASME B31 series and if the are "similar" to those "SA" materials, are assigned an "S-Number" by ASME IX; see QW-420.2.
This is very similar to filler metals; for example A-5.18 vs. SFA-5.18. Did you know that ASME purchases the rights from AWS to use the filler metal specs we find in ASME Section II, Part C? It's true, that's why you will read at the top of almost every one of the SFA specs that the spec is similar to or identical with AWS A-X.XX. Naturally, if adopted for use under Section II, Part C, ASME may put their own twist on things; additional testing, marking requirements, etc.
To close the loop here, I do not believe impacts are required for your project, just as we decided last Friday. Good luck!