It is an interesting question. It should work to give around a similar level of SQ increase as with the ER4P but be less amp dependent overall. But hard to get an actual experience since those who have the S-adapter would already have the P and thus little need for the HF5. Also those who have the HF2 for its purpose would not have/use the adapter with those. Maybe at a meet somewhere the combo will be tried out.
i have used an er4 p to s adapter with many many phones for over a year. it does not work universally well, but often is so good. that said, i have not heard the hf2 or 5. but, i love my p to s adapter so much.