http://www.hebrewbooks.org/pdfpager.aspx?req=37946&st=&pgnum=464
I think R’ Moshe is learning (and the lashon of the Rambam sounds this way) that the added kedushah is not b’etzem, it’s a product of the extra chiyuvim. Kohanim, OTOH, have their extra mitzvos because of the higher level of kedushah.