The biggest problem for a tuned 'd' is traction. A lot of its performance potential comes from its extraordinarily high torque number. The most recent 'M Performance' 'd' vehicles from BMW all come with 4 x 4. That's because RWD tends to convert that amount of torque into blue smoke, heat and noise rather than forward motion.
The most recently released M135i iX does 0-62 in 4.7 seconds. That's 0.2 seconds faster than its RWD sibling, despite the extra weight and frictional losses. The gain comes entirely from the hugely improved traction, which more that compensates.
So, the issue for 'd' vs. 'i' in terms of outright performance is always going to be its 0-10 mph performance and flashing orange traction control lights coming out of corners.
Perversely, point to point a tuned 'd' may actually be slower than a standard 'd' as once a tyre breaks traction you have to moderate both power and torque.
An X5 weights just slightly less than the moon, yet in Sport mode will beat almost anything on the road over the first 100 meters, down entirely to the massive traction available through its 4 huge 285 & 315 width tyres.
Last edited by SteveC; 03-17-2013 at 04:17 AM.