It appears that the Xtreme firmware update for the Plextor M5 Pro is little more than a marketing ploy. While Plextor does indeed get in the vicinity of 100.000 4k read Iops, the SSD is not much faster in any of the other benchmarks. In some tests it's even slower with the new firmware than before.
The Plextor M5S is about as fast as the Plextor M3. That's not surprising since the controller and flash memory are the same, and the firmware probably is quite similar. However, it is quite affordable, and since the reliability is a proven fact, the M5S is definitely one of the SSDs worth considering.
Plextor M5S 256GB