Despite its hefty payload, the new single-turbo, aluminium-intensive, direct-injection engine is able to impress. The merest hint of lag is swept aside by strong, linear power delivery between 2000rpm and the 6500rpm red line.
As revs rise the engine note falls happily on the right side of sporting, yet is also eternally smooth and fades on a constant throttle – even showing 4000rpm in fourth at 70mph, the engine is hushed; in fifth or above, its silent.
The transmission is breezy rather than bellicose, even in Sport mode: neither the reaction to clicks on the positive-feeling magnesium paddles nor the shifts themselves are rapid. Slickness is the upside.
The Q50 Sport can only be had with Infiniti's unique steer-by-wire system, paired with a switchable lane-assist function. You'd arguably need to live with the latter to assess it fully, but the steering itself – adjustable for both weight and speed – is best in the middle of three settings.
Its silkiness cossets, but feel is minimal, and by defaulting into stark lightness at lower speeds it can surprise you. Keen drivers would almost certainly prefer the standard electric rack-and-pinion setup.
Double-piston shocks help produce a ride that's generally very forgiving, yet body control is tidy, too. You can therefore conquer twisting country roads with both pace and comfort, but the 3-series' sports car-like dialogue transpires from neither seat nor tiller.
As always, the interior is neatly trimmed, well-equipped and ergonomically sound, if short on style. Space is good, though rear headroom is borderline for six-footers and near-absent for a fifth noggin.
Standard kit is generally better than in the M Sport but not the AMG Line, both of which include stiffer suspension and more extrovert body kits, if that's your predilection.
One big differentiator is that you must spend £2760 on a multimedia pack to get sat-nav in the Q50 Sport; it's standard on the Merc and £1300 on the BMW.