Weight may be on the RS4's side--and, yes, it does feel strange saying that in the context of a 3771-pound vehicle--but it takes a walloping when it comes to standing starts. All-wheel-drive traction on a dry surface isn't much of a benefit in these disciplines, and whichever way you approach the manufacturer-supplied figures, the RS4 just isn't as fast as the M5 and the worryingly cogent E63. Even so, 0 to 62 mph in 5.0 seconds means this is a very fast car. But the BMW's mark of 4.8 seconds belongs in an even more grown-up paddock. And the next time a Porsche 911 driver considers tangling with an E63, he would do well to remember that the driver of this particular family holdall has access to 4.6-second 0-to-62-mph runs at the stretch of a right leg. Simply shift into D, depress the accelerator, and go. The first time I floored the E63, I had to stop, get out, remind myself it really was a wagon presenting such potency, and then resume. Performance of this magnitude deployed in a humble wagon feels wonderfully irresponsible in a way 507 hp housed in something low and penile never could. Loading it with people and junk and then finding out what lurks in front of that gearbox is like discovering that your grandma is a crack addict.
Keep to the straightaways in the Benz, and it will make you question the other cars' claims to ultimate-wagon status. Being the similar pair in size, and archrival brands to boot, the E63 and the M5 have a grudge match to settle. And, for all the BMW's undoubted provenance, the E63's brutal simplicity exposes the M5 at first. The E63 is a devastating plug-and-go device, its engine sounds fantastic, its cabin is intuitive, its body is easily the most practical, and it looks plain dangerous.
Whereas the Merc's performance arrives virtually unannounced, you have to extract the big numbers from the M5. Its V-10 needs more working than the Mercedes V-8, but beyond 5000 rpm it summons an urgency that no other Labrador-lugger could hope to match. Each time I shifted from second to third at 8000 rpm and watched the head-up speedometer click into triple digits, I completely forgot that I was driving a large wagon, because the very nature of the experience, not to mention the thrust on offer, didn't tally with the car's body style.
Unsurprisingly, the M5 is very good at going fast. Its spring and damper rates are better tuned to the rigidity of its body shell than are the E63's--at road speeds, it feels the same as an M5 sedan, which is quite an achievement. Such composure sat well with Boz, who gave the car high marks for ride comfort but found the chrome load fasteners rather cold on his behind. Priceless feedback, as ever.
The Mercedes is noticeably oversprung and overdamped: the primary ride is as good as the BMW's, making it as comfortable at freeway speeds, but jagged edges harshly ping through the E63's cabin, while the M5 smooshes them under its Michelins. And this is with the softest of the three available damper settings. Boz was unimpressed with the general jiggle out back, but he enjoyed the spacious surroundings. The M5 also steers better with its faster, more direct rack.