For information sake I have 16gb of DDR3 memory at 2400Mhz and an i5-4690k clocked to 4.3Ghz
On the BMW test I found this to be my best tile size
576x 576y = 51.27 seconds
either side of this (512 or 600) yielded between 52 and 53 seconds and it diminished from then on. So that might be a good size to aim for if you have a 980Ti 