M3 may be the culmination of the Munich maker's engineering prowess in terms of a lithe, powerful compact sports coupe, but when it comes to a full-sized coupe (or convertible) with all the luxury trappings, few car manufacturers can match the accomplishment that is BMW's 6 Series.

Sure, Mercedes-Benz's SL coupe-convertible does a fine job of being all things to all people, but as a coupe the 6 Series remains the more focussed vehicle. The BMW also happens to be around $100,000 cheaper than the entry price of a V8-powered SL.

So it's somewhat surprising that after less than two years on the Australian market, BMW's born-again 6 Series nameplate has come in for a substantial upgrade.

In true BMW style, the 2006 revisions are headlined by a stonking new powerplant - in this case a 4.8-litre V8 that betters its 4.4-litre predecessor in terms of on-paper performance by around 10 per cent - without increasing fuel consumption.

While that's an achievement in itself, on the road the 650i convertible feels a lot quicker, faster and more driveable than the figures suggest.

Fire up the new 6 Series via its somewhat gimmicky new starter button and a far beefier exhaust note is the first giveaway that a more formidable V8 lurks within.

Of course, that aural sensation is magnified in the fabric-roofed 650i convertible, but there's no doubt the 650i has a far more pleasing German V8 exhaust note than the same engine in either the 550i or 750i sedans.

Brush the accelerator pedal and initial impressions are confirmed: the 650i leaps forward considerably more enthusiastically than its 645Ci forebear and offers a more ferocious power delivery all the way to its 6500rpm redline.

In fact, with performance figures that aren't far off what the previous-generation E39 M5's 5.0-litre (M-tuned) V8 offered, the new 4.8 makes the 650i every bit as satisfying as one of our favourite sports sedans of all time.

BMW's bigger-bore V8 may not match the outright performance of the new-generation 5.5-litre V8 that's currently filtering down to top-spec variants within the Mercedes-Benz model range, but it's in a similar league, offers outstanding tractability and fuel consumption, and sounds beaut to boot. And then there's the M6's V10...

That said, such is the intoxicatingly effective acceleration of the 650i that, try as we did, we couldn't replicate the convertible's claimed average fuel consumption figure of 11.7L/100km.

In fact, after an enthusiastic 450km drive through the south-east Queensland hinterland, we averaged more than double that (just under a thirsty 25L/100km), which I guess is the price one pays for performance.

Beefy new engine aside, the 6 Series' other techno advances are well worth a mention.

First there's BMW's new head-up display, which cleverly presents an image of a digital speedo and other basic information (such as abbreviated navigation directions and whether stability control is deactivated) on an invisible screen directly in the driver's line of sight.

Far from distracting, it proved a boon on the somewhat rapid launch drive, as did the 650i's standard Dynamic Drive, which provided our convertible with a vastly more compliant ride but even better roll control than the firmly sprung 540i sedan we drove on the same day. In short, BMW's variable stabiliser bar technology is a revelation to those who thought a plush ride and suspension control were mutually exclusive.

Other aspects of the 6 Series remain the same: the two rear seats are sumptuous but torturously tight for adults, the electric seat folding function works intuitively, the push-button roof mechanism is a masterpiece of design that never fails to draw a crowd, the active steering provides unprecedented levsl of convenience in the carpark while the thick-rimmed tiller remains consistently well weighted at speed, and the separate buttons for all four windows and the electric glass windblocker are a neat touch.

While the 650i's exterior design is a subjective matter, it's difficult to pick holes in what is an accomplished large coupe or convertible.

Yes, the 650i could be lighter and more fuel-efficient, and it lacks the steering wheel-mounted gearshift buttons of its rivals.

But a combination of top-shelf build quality, cutting-edge technology, a rock-solid ride/handling package, relatively accessible pricetag - and now a blistering new level of performance and driveability - makes the 2006 650i an even more highly accomplished luxury performer.