If you do enough measurements on that new prompt then I don't see why this shouldn't be a paper. People overestimate the value of "grand developments", and underestimate the value of actually knowing - in this case actually knowing how well something works, even if it is as simple as a prompt. Compare with drug trials: Adderall only differs from regular amphetamine in the relative concentration of enantiomers, and th…
I think this depends a lot on the "culture" of the subject area. For example in mathematics, it is common that only new results that have been thoroughly worked through are typically "publish-worthy".