I was looking at some of these models as well. Except I was looking more at the Benz E-Class more than the Infiniti M45.
From my experience, the E-Class and the GS both drive the nicest. All those people that claim that Lexus is a glorified Toyota (not really a bad thing, IMO) have not been in a Lexus lately. Both are smooth. The GS is definately faster compared to it's Benz counterpart when you compare the 430 engines. Since I am comparing a GS400/430 to the 96-01 Benz E class bodystyle, the E looked outdated compared to it. The current E is much better looking, but when you compare a new fully loaded E430 compared to a fully loaded GS430, the price is at least 10 grand apart, with nearly equal power and quality, plus you get Toyota/Lexus reliability. The current GS bodystyle has been around for a long time, but it doesn't get old. You have new cars today that copied the lines of this GS.
The Bimmer's biggest strength is it's handling. I don't think it drives with the same comfy smoothness of a GS or a E, but the 540i is a very fun car to drive. BMW's reliability recently has taken a nose dive. Even though a large chunk of that is due to the big 7 series, it makes you skeptical. I also like the styling of the car. But I like the GS style more and the GS feels more like a luxury car than the Bimmer. Quality is why I only buy Toyota and anything associated with it, so as nice as it was, the Bimmer was not an option for me after I read about all those issues with other BMW's. There are all kinds of horror stories about brand new 745's with big time electrical issues. If their flagship has these many problems, then what are the chances that their mid size sedan is the best built car in it's class? If you are used to Toyota's reliability, then I would not reccommend a Bimmer. The 00 and up 3 series and the previous 5 series are still 2 of the best looking cars I've seen.
The M45 I did not really look at. First off, I barely knew that it even existed when I was looking for a car because no one buys them. I wouldn't be surprised if Nissan/Infiniti overrated their engines because that's their marketing idea: horsepower. Everytime you see a Nissan ad, it almost always mentions the horsepower first. I think the late 90's model Q45's had higher rated power than the 97-00 Lexus LS, but the LS was quicker than them with about the same weight differences.
As for the stock GS4 running 13.9, I don't know. Maybe with perfect whether/road conditions and a good driver, yes. Other than that, I'd say mid 14's is reasonable with low 14's being achievable. The car is definately better than high 14's though, from my experiences.