I think if you consider MPG, you should choose Lexus but I believe this premium Nissan has much more fun.
Yes. My GSF sounds much better but I'm not very excited about it except the first week. After that I think it's cosmetic and a little bit noisy in daily drive... (I'm an engineer. I like effective more than cosmetic.)
People buy V8, V10, V12 for it's performance and also for the noise it makes, the pure engine note that you can't find in force induction engines.
I wonder why you bought GSF after tested so many cars. If you are serious about efficiency and want something bit quieter, you should consider cars that equip with technologies that recycles energy. Such as electric cars, or cars runs with hybrid engine. They are designed to lower fuel consumption considerably, quiet and still got plenty of performance to offer.