ummm...to be honest...no.
people are bound to jump on me when i say this, but the theory that "its quality not quantity" is, as far as the HSC exam is concerned, utter rubbish *hides from peeps throwing BOS booklets at head*
ill tell you what my dad, a senior english HSC marker, told me: what you need is quality IN quantity. remember, you are, in essence, competing against a lot of very talented english students. it's not enough to write something that is high quality but only short - because you're being judged against people who write quality pieces but have managed to maintain that standard over a length - it is this ability to SUSTAIN your technique that shows the skill level the markers are looking for (i think you'll find actually, that "sustained piece" will be part of the question rubric).
the theory the markers adhere to, whether fair or not, is that if it is not at least 5 pages, whilst it may still score well, it cannot be worth top marks. remember, this is your chance to show your level and scope of literary skills, dont waste that opportunity by not writing a piece that shows you are able to develop motifs and themes and demonstrate technical control over a sustained piece.