The engine is definitely the star attraction. The big V8 has an abundance of brawn – generating peak torque from just 1800rpm – and also delivers plenty of character.
It’s well-mannered under gentle use, although producing little more than a muscular hum, but it’s also happy to lift its skirts at a moment’s notice, pulling cleanly all the way to the 7000rpm redline and producing a nicely judged amount of eight-cylinder snarl when asked to do so. The gearbox helps out with intelligent part-throttle shifting to help wake the engine and impressively rapid ratio swaps at higher commitment levels. In terms of straight-line pace and soundtrack, it comfortably outguns the Mercedes-AMG E43’s turbocharged V6.
But the M550i struggles when asked to deliver on the rest of the sports saloon experience. In essence, it’s a very fast version of the standard 5 Series - a car that already feels a fair bit easier-going than its predecessors - rather than a not-quite M5.
Suspension is soft and, despite the presence of adjustable dampers, struggled to maintain body control over rougher road surfaces, even in the firmest Sport mode; there’s a surprising amount of body roll as well. Grip levels are predictably high, but the handling balance feels nose-heavy under hard use and, although accurate, the steering allows pretty much no sensation regarding the road surface through its syrupy assistance. It’s effortlessly fast and very secure, but there are none of the push-harder vibes that the presence of an ‘M’ badge might legitimately lead you to expect.
Of course, the M550i still has all the other virtues of the current 5 Series. The cabin is beautifully finished and packed with standard kit, the tech all seems to work well and is presented beautifully, with crisp animation of the large central display screen when you change modes.
Our test car also came with the semi-autonomous CoPilot active steering system, which proved adept at following US freeway lanes, although the lesser lane keeping assistant – which can apply steering torque when it thinks it needs to – seemed considerably less able to distinguish road markings and was soon switched off.
Cruising refinement is almost freakishly good, with the M550i being no louder or harsher than a standard 5 Series at the sort of rapid cruising speeds it seems to have been engineered to deliver. Economy is also entirely respectable, returning the equivalent of 27mpg even under hard use.