In theory, the Mercedes-Benz CLA45 AMG should be exactly the same to drive as the rabid and deeply excellent Mercedes A45 AMG. Because despite its sweeping new junior CLS lines, the CLA is actually identical to the A45 AMG beneath the skin.

That means you get the same 355bhp, 2.0-litre turbocharged four-cylinder engine as you do in the A45, the same crisp but direct electric power steering, the same seven-speed dual-clutch automatic gearbox, even the same spring rates for its new strut and multi-link suspension system.

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We liked the CLA45 AMG a lot, but it doesn’t feel as sharp as the smaller A45 AMG

Thus, and despite it weighing a piffling 30kg more than the A45 AMG, Mercedes even makes the same performance claims for its two new four-wheel-drive, entry-level AMGs.

The CLA45, just like the A45, will hit 62mph from rest in a claimed 4.6sec thanks in part to its 'race start' launch control system, and it’ll reach the same 155mph limited top speed, having hit 100mph in approximately 11.0sec en route, making a delicious noise in the process – particularly if you go for the optional AMG Performance Exhaust. 

So although a dinky little four-cylinder coupé may not sound like the kind of car that would normally wear Mercedes' most prized of badges, times are a-changing, even at AMG. And in the process, the raw performance of the cars is suffering not one bit.

And when you look at what the CLA45 AMG offers, at least on paper, it’s hard not to be impressed by the numbers it generates – except for one, which we’ll come to in a moment. The headline figures are as follows: 355bhp, 331lb ft, 0-62mph in 4.6sec, 161g/km of CO2 and 40.9mpg on the combined cycle. That is an extraordinary series of statistics by any reckoning, and pace you can really feel on the road thanks to its grippy four-wheel-drive chassis. On wet UK roads in particular, we doubt there are many cars out there that could keep up with it. 

When you then dial in the combined effects of a cabin that is both well made and surprisingly spacious front and rear, plus a decent-size boot and a level of on-road sophistication that’s unprecedented from a four-cylinder, four-door coupé, it’s not difficult to see why the CLA so impressed those who drove it at its launch.

Yet there are two factors that count against it, one of which is rather more important than the other. The first is the slightly odd realisation that it doesn’t drive as crisply as the hatchback on which it’s based.

The only difference between the cars, according to AMG’s amiable engineering boss Tobias Moers, are the damper settings, the CLA’s being fractionally softer in both bump and rebound to make it feel that little bit more chilled than the hatchback on the move. Which is fair enough in theory, even if the reality seems to be 10 per cent less zing pretty much everywhere.

The second difference is the price. The CLA45 AMG costs more than £40,000 in base form. Throw a few options at it, in other words, and maybe go for the smarter-looking 19-inch wheels that were fitted to most test cars, and you will be looking at £50k-plus. For a Mercedes A-class. Blimey.

Admittedly the CLA45 AMG neither looks, goes nor drives like your average A-cass, but in the end that wincingly high price could well be its undoing. That, plus its mildly oddball status as the world’s fastest-ever four-cylinder coupé that just so happens to have four doors.

Still, that price has not put off early buyers, quickly selling out of its initial right-hand-drive production run (most cars being well-specced, often in excess of £50,000, at that) and resulting in a long wait for those who want to sample a car that could go on to change the way we think about our more desirable modes of transport at a stroke if it maintains its early momentum.