I think dopie is just going to have to drive some sedans. IS-F and GS aren't even in the same category, so they're not that comparable. The GS is a great car (brother in law owns one), but it is somewhat boring. It would be great to cruise around in, way more comfortable than my old IS350. This is just my opinion, but the IS is too much of a compromise if you have a family. They just put the back doors on as an afterthought, it's really a 2 dr. coupe and a small one at that. Do you want less interior room than a Toyota Corolla?
M5 is slower than a CTS-V; but it also drives completely differently. Neither one is better, but it's going to depend on the person which one they prefer. When we're talking about that kind of power, it starts to get somewhat meaningless. 99% of the time, you won't be able to really go all out in a car like that. It's easy for everyone to internet race these cars, but you're just going to have to go drive them.
Later this summer, I'm likely getting an E93 M3 since I already have the Infiniti for practicality. For that kind of money, I could get some faster cars. But it's not all about speed; I think that it's more about how fun the car is to drive. Currently I drive around the city in a Protege5 since I have a long 1.3 mi. commute to work. This thing has a whopping 130 hp. Surprisingly, it's way more fun than one of my previous cars with north of 450 hp and a huge turbo. It's because I can drive it flat out all the time while if I did that in a high hp car, I'd wreck it.