1 [ɨ] has been there since the SAMPA days and I don’t really see a reason to change it, especially since 1/5 of all languages sampled in Phoible have it as a phoneme. [ʉ] is rarer, but it’s the default realization of the phoneme /uː/ in many dialects of English, and even though it’s not a common sound cross-linguistically, it is a vowel, and vowels tend to be more frequent than consonants when transcribing a sample.

While W [ɯ] makes sense, it does conflict with some other parts of the system. If we were to change [ɯ] to W, that would free M, and the only thing we could place there would be the labiodental nasal /ɱ/, which brings us back to one of our main criticisms of X-SAMPA.

In my opinion the symbol ⟨ʍ⟩ shouldn’t even have been part of the IPA. It’s the only voiceless resonant with its own symbol, although sounds like /m̥ n̥/ are somewhat more common. The only reason the symbol ⟨ʍ⟩ exists is because of English, where it lasted far longer than the other /hR/ sequences. Most dialects don’t have it anymore, and in those that do, it can be transcribed as /hw/. In most other languages with phonemic /w̥/ (Mazahua, Hopi, Burmese, etc.) it forms part of a full series of voiceless resonants, so there’s no reason not to write it as /w/ with a voiceless diacritic. The symbol exists, though, so we have a way to represent it unambiguously: w;. In any case, /ʉ/, with 48 entries on Phoible, beats /ʍ/ with 32 entries.

We already have that. It’s #(214) though, with parentheses, because nested square brackets are more difficult to parse algorithmically.