In one sense - a trivial one, perhaps - this is the last of the high-performance Imprezas. In 2010 Subaru decided it would use that name only for diesel and non-turbo petrol cars, with the result that what would have been known as the current Impreza WRX STI is now simply called the WRX STI. The car reviewed here was launched before that policy was implemented, and is therefore a living fossil from a previous era.It's possibly the greatest product of that era. Conceived and developed by Cosworth rather than Subaru itself, it uses the familiar 2.5-litre flat-four turbo petrol engine which for this application has been so extravagantly modified that it produces a maximum of 395bhp - a third as much again as it does when fitted to the WRX STI. Top speed is limited to 155mph, though you can bet your boots the car could go faster than that in unrestricted form, and the 0-62mph time is quoted as being 3.7 seconds.I hope you are deeply impressed by that last figure, but it only hints at what the car is really like. In the same way that you can read about drag racing, or watch it on television, for years without having the slightest idea of the sensory overload you experience at a meeting, there's no way of conveying the reality of the Cosworth Impreza without strapping you into it and giving a demonstration. But I'm here to try.Let's start with that acceleration. The funny thing is, it doesn't feel quite as dramatic as you might have expected. One reason is that there is absolutely no fuss to it whatever. With the 395bhp being divided among four wheels, and the suspension being quite brilliantly set-up (as we'll discuss in more detail later), there is no sense that the rest of the car is struggling to cope with what the engine is throwing at it. It's almost as if it could deal with another 50bhp without any great trouble.Reason number two is that the acceleration, formidable as it is, comes in short bursts with sizeable gaps in between. Once you've reached maximum revs in one gear (preferably without hitting the rev limiter, though you need to be quick on the draw to avoid doing that) you have to change into the next one, and that's going to take a while because the change can't be rushed. On one occasion I crunched the synchromesh on the way into fourth, and I didn't think I was shifting all that quickly.In the time it takes to make that change, the turbo falls below its useful operating speed, and it takes another few tenths to regain it. So a flat-out straightline run goes something like WHOOSH, change, lag, WHOOSH, change, lag, WHOOSH, change, lag, and so on until you decide you've run out of tarmac, nerve, legality or whatever commodity allow you to do this.Despite the official 0-62mph figure, this obviously isn't a very efficient way of accelerating. The mind boggles at the thought of how fast the Cosworth Impreza might be if it were fitted with a suitably robust twin-clutch semi-automatic gearbox whose changes would be so fast that the turbo didn't fall asleep during them.When you're on the move and starting to accelerate from relatively low revs, the engine starts to pick up properly from around 2500rpm and seems to me to jump three or four times to new levels of performance until the dams fully open at around 3500rpm.At lower revs, and on a light throttle, there is much less power, but still enough to make good progress, and the Cosworth Impreza proves to be a remarkably docile machine which you could happily give to someone after their first driving lesson (possibly having taken the precaution of placing a brick underneath the throttle pedal before doing so).In these circumstances it's also relatively comfortable. The 245/40x18 tyres make the ride harsher than it might be, but the suspension is surprisingly soft, demonstrating beyond all doubt that overly stiff springing and damping simply isn't the way to go in a high-performance road car.If anything, the front end seems underdamped in gentle motoring. The mild bouncing that results from this makes it seem at first as if something has gone badly wrong, but in hard driving it becomes clear that nothing is wrong at all. The Cosworth Impreza is simply extraordinary in fast cornering. It is quite definitely not a point-and-squirt car, to be coaxed gently round bends and unleashed only when the road straightens up.On the contrary, if you can arrange things so that the engine comes in at full blast in mid-corner (which takes quite some commitment, since you need to have the pedal buried in the carpet well in advance to account for the turbo lag) the effect is that the rear end helps to keep the car turning until you reach the exit.It doesn't push the car sideways - I can hardly imagine what you have to do to make this thing spin on the public road - and there isn't the slightest hint of understeer. As an example of maintaining cornering balance in extreme conditions, this is little short of astonishing.Only a very few Cosworth Imprezas have been built, and the fact that they each cost £49,995 hasn't helped their sales in the current financial situation. But although this is not an attractive-looking car, nor one that anyone actually needs, its exceptional abilities mean that it's worth every bit of £49,995, and the people who are able to buy one because it didn't sell out as quickly as expected can count themselves very fortunate.