Halliday & Matthiessen (2014: 422):
Postmodification is of one type only, namely comparison. As in the nominal group, Postmodifiers are rank-shifted, or embedded; they may be (a) embedded clauses, or (b) embedded prepositional phrases. Examples:
(a) much more easily [[ than you would have expected ]]
as grimly [[ as if his life depended on it ]]
too quickly [[ for us to see [[ what was happening]] ]]
not long enough [[ to find my way around ]]
(b) as early [as two o’clock]
faster [than fifteen knots] …
This is the only instance of embedding other than in a nominal group. All other embedding in English is a form of nominalisation, where a group, phrase or clause comes to function as part of, or in place of (ie as the whole of), a nominal group.