[vinay427]

> From what I can tell Hindi phonemes are a superset of English phonemes.

This isn't exactly correct. There are a variety of allophones that many Hindi-native speakers use in their English that aren't used by most native English speakers outside of India such as the retroflex t, trilled or flap r, and others. There are also others missing from Hindi entirely such as /ʒ/.

See phoneme tables for English and Hindi/Urdu (in IPA for instance) for more details.

I agree with your premise, in any case, that Devanagari describes far more than the English/Roman script.