re: the GS400 - From a dead stop it would be a good race but I think you'd get the jump on him for the first couple of gears and he'd have to try and reel you in - which I don't think he could in time...
From a roll on the freeway - I think we would get slaughtered... Those cars can really push you back in your seat once inertia is taken out of the equation. :-)
I worked as a wrench and eventually a service consultant at a Lexus dealership for 4 1/2 years so I got to "test" a fair number of the cars that made it through there.
Believe it or not, the luxo-barge LS400's of yore could really blitzkrieg down the highway if you felt like it too. Scary stuff when those ~4500 lb. cars are bookin' it on the 4-lane!
Also, if you managed to get him on a twisty road the GS400 would be meat. Those things are cruise missiles - not canyon carvers like the WRX is. That might be a suggestion. :-)