Make sense, but the GS400 was more expensive not to mention takes more gas too :(
About the more gas in the 400. My wife drives between Memmphis TN, and Ashville NC, on one tank of gas in her 2000, LS 400 with 90,000 miles on it. It's over 500 miles. I think her best time is under 9 hours. She gets 27 miles per gallon at between 70 MPH and 80. I tried that in my 2002 GS-300 Sport design trying to better her average. Went very easy and cruise when I could. My best tank was 27.4. So the diffence in the 300 and the 400 aren't going to break the bank. Around town they are very close, not enough to cry over. The 400 engine never breathes hard.
After you muck up a highly engineered car, made by some of the best engineers in the world, it will cost you more than a 400. And you probably won't improve anything. Like someone said, it could be worse.