Right from the start, both of the SM951s from RamCity outperform the SM951 from Lenovo. Normally we'd expect the higher-capacity SSD to perform better thanks to its increased parallelism. But in this case, the difference between the 256GB and 512GB models is large enough to fit a SATA 6Gb/s SSD in. You could park the peak performance of an 850 Pro in the gap!
It appears that Lenovo limits the SM951 for its Ultrabooks. This would reduce power consumption and the amount of heat generated inside its compact chassis.