have u investigated import fees and stamp duties?
cos i think once u get above a certain value of goods, the amount u pay for bringing these in will go through the roof... unless u get them all sent from US individually, to different addresses.
Sorry to poop the party but i think this issue has come up before when group-buying from countries outside of Australia/NZ..
Its jsut that 5 sets of pistons will be valued at over $3000 so the taxes ul pay will be lets just say 'large'.
Dont quoute me on any of this, as im no expert, just thought id mention it if u hadnt thought of this yet.
could even try asking importers for advice, or AusTrade website, i think they give advice for import/exports..