What we seem to be looking at is an MP4-12C with revised styling to more closely resemble the P1. Which is a good thing, because to most eyes, the P1 is a far better-looking car than the 12C, whose appearance – finalized before design director Frank Stephenson arrived on the scene – has oft been derided as derivative. The 650S also features an updated interior, but best of all, it's said to pack more power: the name hints at a 650 metric horsepower (641 hp) output from what we can assume to be the same 3.8-liter twin-turbo V8 that powers the 12C (which currently offers 616 hp) and the P1 (727 hp before the 176 hp electric motor comes into play).
What remains to be seen at this point is whether the 650S you see here will eventually replace the 12C or continue alongside it indefinitely. The prevailing wisdom seems to be that McLaren will offer both and see how sales pan out. Scope it out in the (unfortunately grainy and low-res) leaked image gallery above and the walkaround video below, and watch this space for the official reveal.