Well..... When the GS400 came out in 1998, one of the high points that Lexus mentioned of their 300 horsepower 4.0 liter V8 was that it was more economical than the old inline 6 that it replaced (which was in the old GS300 and is pretty much the same as the current engine but without the VVT-i.).. So to answer your question, the engine in the LS400 (4.0liter 290hp V-8) is more advanced (with Aluminum Block) than the engine in the IS300..... BUT really I think the main reason for the poor fuel economy in the IS300 is its final drive ratio.....
1> Final drive ratio?
2> Inefficient engine?
3> Extra rich at first to get primary catalyics up to operating temp fast (to get LEV status)?
4> High rolling resistance performance tires?
5> Aggressive e-Throttle setting?
Someone asked a Lexus tech who replied "it is a performance car..." Well, OK, whatever.
Your guess is as good as mine.
The Nissan Maxima (I think with 222hp) performs a tad better than the IS300 yet gets much better gas mileage. It has a few things which help:
1> Front wheel drive (more efficient drivetrain)
2> "Micropolished/Microfinished" engine parts which are more efficient.