I dunno, if I were to buy an M5 the only second car I would consider would be a dedicated track car: light, nimble, stripped down, and not too expensive so I wouldn't be miserable if I wrecked it. If you're going to have another car for "fun but non-track" driving, does that mean you'll be relegating your M5 to nothing but driving up and down city streets and freeways? I've seen a lot of M5/M6 owners do that and it always makes me sad and confused; there are more comfortable and less expensive cars for that (like the 550i) and it's a waste of an amazing car IMHO.
I know these aren't exactly track cars (though they seem to hold their own reasonably well), but even if your M5 won't see a track, hopefully you'll at least be taking it out on twisty back roads. But if you want something faster for that purpose and you're contemplating a 458, might as well throw in a GT3RS, GT2RS, and MP4-12C -- though having caught a ride in an MP4-12C at a track event piloted by an instructor, I can tell you that people who own those cars and never take them to the track have no idea how much of their car's value and fun potential they're leaving on the table. The 12C properly driven in the proper environment completely recalibrates your sense of "fast".