sorry, that's incorrect.

460.9 lbs - LS1 (5.7L) Manual with oil and flywheel
405.5 lbs - LS1 (5.7L) Auto with oil and flexplate

For comparison, the 1998 GT total dressed engine mass (dry) for 1998 is listed as follows:

Manual trans: 529 lbs
Auto trans: 492 lbs

now, these aren't the greatest sources, but it's what i could find real quick.

and what do you mean by inefficcient? hp/liter? or hp/lb?