http://www.camaro5.com/forums/showthread.php?t=13056
Basically the reason for the disparity from what I gather is "Pointy cams, less duration, have greater torque at low RPMs but less HP" and other reasons.
The Auto runs as a 4 cylinder to save fuel under certain circumstances so I'm told as well *shrug* I'm sure that would involve more 'parts'.
Random but the manual does not avoid the gas guzzler tax in Canada :( I'm sure that worries you in Vegas... lol