The IS300 has a drag coefficiet of 0.29, which is among the best in cars of this type.
Since the car already has smooth panels underneath to minimize turbulence from chassis components, and the body design is already quite aerodynamic, I doubt you could improve CD by any significant amount.
Here are the specs and a pic of the Team Lexus car, and it really doesn't look too radical.
http://www.clublexus.com/index.php/article...view/2135/1/11/
Drafting on the highway would also seem impractiical because you'd have to run so close to the other car that it would be very dangerous. The best thing for highway mileage is running at a steady crusing speed and not "getting on it" a lot when passing, etc. I try to stay far enough behind trucks to stay out of the turbulence, when conditions are such that I can't just go around them.
I've gotten 26MPG 1500 mile round trips from AL to Ohio using cruise control and in light traffic. When the traffic gets heavy, there's not much you can do about it. I usualy cruise at 75 to 80 MPH in my 2002 E shift IS. Keeping all the windows closed and running the A/C is also good.
Tire pressure also makes some difference. I run 34 normally and raise to 36 (cold) for long trips.
Hope this helps.