Richard, did you have a look at that Ilford page? My reading is that they describe 3 equivalent approaches in the section on the right of page 3, specifically equating with each other: 1. Fixing up to 10 sheets of 8x10 in 1l. 2. Fixing in a two bath fixer. 3. Fixing up to 40 sheets in one bath when using washing aid.
Since 10 sheets per litre is what you describe as archival, and since they suggest that is equivalent to 40 sheets with wash aid, it would seem to imply that 40 plus wash aid has to be as archival as 10 without it. Digital Truth seems to suggest that too.
Perhaps I am not reading that correctly...