Weighing 50kg less than the road-going model and featuring 62kW more power, the GTR boasts a massive 735.5kW combined output (or an even 1000 metric horsepower) from its 3.8 litre twin-turbo V8/electric assistance motor, all of which is sent to the rear wheels.
The electric motor itself is a lighter version of the road-going P1's motor and is optimised for motorsport. McLaren doesn't go into specifics, but says that key powertrain features designed for "highway use" have been removed to save weight and improve performance.
A titanium alloy exhaust system also helps shed a few kilos - 6.5kg, to be exact.
The power split between the petrol engine and electric motor is a solid 80/20, with the turbo V8 providing 588.4kW and the electric motor providing 147.1kW.
With the electric motor able to deliver all of its output instantly, it helps fill in the torque "hole" experienced when the V8 at low revs and off boost.
Further weight reduction comes from the substitution of all side glass with fixed polycarbonate windows (the driver gets a sliding panel), while the glass roof panels and engine cover are turfed in favour of carbon fibre replacements.
Meanwhile, the road car's retractable rear wing is replaced by a much larger fixed-height wing, which sits 100mm taller than the road car's wing when at maximum extension.
Working in conjunction with a prominent front splitter and deep side skirts, the wing generates 660kg of traction-promoting downforce at 241km/h.
The wing can also be trimmed by the driver to produce zero lift, handy for long straights where downforce becomes less important than reducing drag.
Aiding handling is a front track that's 80mm wider than the road car, plus a 50mm lower ride height. The 19-inch forged alloy centrelock wheels are shod with sticky Pirelli slick tyres, granting the P1 GTR racecar-like grip.
Only 375 will be sold, and McLaren will only offer them to existing P1 owners.
The cost in the UK will be an eye-watering AU$3.9 million, making the P1 GTR one hell of an expensive track toy. That's right, the P1 GTR is not road-legal in most countries.
At least those forking out for the P1 GTR get a little more value for money in the form of entry to the McLaren P1 Driver Programme.
Similar to Ferrari's XX scheme, the McLaren P1 Driver Programme puts P1 owners through a rigourous race driving school and includes time behind the wheel of a simulator, a testing session at Silverstone then a race session at Catalunya.
Entry to the programme also includes a custom seat fitting, ensuring even the most generously-proportioned billionaires will feel comfortable behind the wheel of their P1 GTR.