Well, the W8 motor is heavy.... 460#. can probably pull a few # off, however that is nearly 80# more then the LS4. However it is much shorter, only about 17-18" long vs the LS4 at 23" But it is really tall and wide compared to the LS4
also, stock output is 271 hp from the 4 liter. with some minor work, you can get 300hp. while it can be boosted, the very narrow rod bearings and the split throws on the crank limit the power to about 450 before things have an un-scheduled rapid disassembly. Normal service items like the thermostat, etc are ultra-expensive.
here is a teardown.