It's a scientific fact that slats, vents and air ducts add precisely 40hp to a car's power output. This fact conveniently explains the output of the Brabus GLA 45 AMG, a 400hp mini-SUV from Merc's semi-official tuning arm.

This is the centerpiece, if that's the word, of Brabus's tuning programme for the GLA. Sure, the German tuner will offer you a modest 'plug and play' upgrade package for your GLA250 and 220 CDI, but it's the GLA 45 AMG you really want to know about.

It boasts what Brabus calls its PowerXtra B45 V-Max kit, and gets aesthetic additions in the form of a rear diffuser, front spoiler and splitter, and larger front air ducts, all rendered in traditionally uncompromising Brabus style.

There are special wheels and performance tire packages, too, and a jazzed-up interior with optional Alacantra leather and LED lighting.

But the really interesting stuff is in the power department. Through unspecified means (though we'd suspect an ECU tweak), the GLA 45's 2.0-litre turbo four-cylinder - an engine also found in the A45 AMG - has been boosted by 40hp and 50Nm to an overall 400hp and 500Nm. It also gains a stainless-steel four-exit exhaust with a butterfly valve, allowing you to choose precisely how much to annoy your neighbours.

Brabus reckons its GLA 45 will manage 0-100km/h in 4.4secs and 270km/h flat out: the, um, 'standard' GLA 45 is electronically limited to 250km/h.

No word on the price of these upgrades, but don't forget the GLA 45 before you start bolting on the Brabus extras.

So, the Brabus GLA 45: good tune, or bad tune?