There is a lot more to a built motor than rods and pistons. So many people have made up to 800hp on the stock shortblock it's not even funny, if your not looking to break the dyno, there is no reason to even put rods in the motor for 600hp, by the time you get rods, machinework, assembly you've got a pretty penny into your motor. I've seen the guts of the fastest 2JZ's on the planets, none of which use stock pistons.
As for the eBay manifold comment, anyone who has seen, heard my car has had no complaints on it. If I bought 10 of them, made the necessary modifications, slapped a Titan or MVP emblem on the item and then sold it for 1200 I'm sure it would be all the rave. :dunno: