And so it falls to the M40i to convince us that the new Z4 is up to snuff. Earlier in the week, we reported that the entry-level Z4 20i, was entirely fine but never likely to spark your trousers alight. The 340hp version needs to do better, especially as there is no Z4M on the horizon. As far as the specification is concerned, the right ingredients are definitely there - and, lest we forget, the car has already beaten the M2 Nurburgring lap time by three seconds.
The M40i's motor is the same B58 3.0-litre inline six found in the M340i and 40i versions of the X5 and X7, and it is technically very similar to the older engine of the M140i and M240i models, which we like a lot. In the Z4, alongside 340hp the turbocharged unit delivers 369lb ft of torque, which passes through an eight-speed automatic and a standard-fit M Sport differential. It's a setup that, on paper at least, looks quite a bit more exciting than the flat four-based one of Porsche 718 Boxster - which is a good place to start.
As with the 20i, our first acquaintance with a right-hooker Z4 M40i is in Marbella on a sunny spring day, so the lessons learned here will have limited relevance to Britain. But you don't need potholes and sleeping policemen to notice that there's a broad spectrum of performance on offer from the top Z4's standard-fit adjustable damping. In comfort, the ride is extremely supple and smooth, while in Sport it's firmer yet never harsh. In fact, the body control we praised in our passive damper-equipped 20i is heightened to such an extent in the M40i that it makes the car feel far lighter than a 1,535kg kerbweight would suggest.
Six-cylinders - surprise, surprise - does it not harm. There's an effortlessness to the M40i's delivery, one characterised by the minimal amount of work required from the gearbox to keep up the pace. All of the same good refinement traits that we discovered in the 20i - provided in part by the Z4's very rigid CLAR underpinnings and an effective wind deflector - are maintained here, only now with the alluring soundtrack of a proper performance engine and a quicker-shifting eight-speed auto that feels better suited to the job. Top up in comfort mode, the motor is quiet but purposeful; top down (folding it electrically takes ten seconds) in sport and it crackles through the exhaust on a lifted throttle. Much, much better.
In the lower gears, acceleration is suitably rapid and the M40i hurtles forward from corner to corner with real intent. On a 21-degree day the rears, which wear 255 section Michelin Super Sports, have so much traction that it takes deliberate flick of the steering wheel on exiting a corner to unsettle their contact with the road surface. Even then, the diff's ability to juggle torque means you have to be a real hooligan to generate a slide; otherwise, the M40i just gets up and goes.
There's next to no pitch or roll on corner entry and the front axle appears to have an answer for everything you can throw at it. Really hustle the car into a bend and the drop-top responds quickly and without complaint, with only that low frequency coursing sound of sticky rubber closing in on its limit illustrating that you are indeed pushing it. Still, the chassis balance is neutral, so getting on the power early is easy, although with so much traction at the rear it only quickens the slingshot out of a bend. There's no Bavarian saloon-esque chasing of throttle to satisfy the senses here.
In that regard, the M40i is a superb point-to-point machine, and that claim for an M2-beating 'ring lap seems entirely believable. But, even with far more power on offer than the 20i, the Z4's ability to make the leap from hugely competent sports car to pant-wetting excitement generator is still somewhat muddy. The joy of driving the M40i is located in its assured capacity for carrying huge speed - but anyone expecting its attributes to feel a little more connected or intuitive - or even just 'fun' in a way that seems fundamental to the package - might find the result frustrating.
Which certainly isn't to discredit the model's wider talent. Indeed, there's probably nothing else in the class with the sort of bandwidth that lets you entirely relax one minute, before credibly rising to the challenge of very fast road driving the next. But you could argue the most expensive Z4 needed to be more than well rounded to crack the status quo among mid-level sports cars. We'll know for sure when the M40i has had the chance to encounter its closest rival on UK roads. Meaning it's still all to play for...
SPECIFICATION - BMW Z4 M40i
Engine: 2,998cc, six-cyl turbo
Transmission: 8-speed automatic, rear-wheel drive
Power (hp): 340@5,000-6,000rpm
Torque (lb ft): 369@1,600-4,500rpm
0-62mph: 4.6 seconds
Top speed: 155mph (electronically limited)