The best and most efficient upgrade, somewhat affordable is called an LS1. But yes, it's called a scavaging effect, you want a certian amount of back pressure.
1. Cold air intake
2. Cat-back exhaust
3. Headers
4. Chip
5. TB Spacer
6. Thermost
before i upgraded to an LS1, i had an 00 4.3 vortec, .030 bore, forged pistons, stock crank, comp cam, 1.7 rockers, Stock ported heads, 5 angle valve job, fitted with oversized valves, 160* thermo, Headers, Catback exhaust, upgraded computer, corvette radiator.