McLaren is a bit nuts, but in a good way. When most of us look at the MP4-12C, we see a damn near perfect car. When the engineers from McLaren look at it, they see potential for improvement. Previously, this has manifested itself in yearly upgrades, but now it's leading to a track-only variant.
The McLaren MP4-12C GT Sprint is inspired by the GT3 and Can-Am racing models, and features much of the same aerodynamic and cooling upgrades as those two cars, as well as a lowered ride height, chassis tuning similar to that of the GT racer, and Pirelli racing slicks. Basically, it's a GT car that's a bit more forgiving for amateur drivers who have no intention of actually racing, but want to spend "below GBP £200,000" on an amazing toy.
There is one way in which the GT Sprint is just hands-down better than the GT3, though: air conditioning. The GT Sprint comes with a lightweight A/C system, and that makes all the difference.