X25-e
FWIW: My 4,1 17" 2.6 MacBook Pro did 220 on XBench (the highest disk test score was 286). I've seen a 2.93 UMBP at 238 with X25-Ms. But I would still recommend the X25-E if you're looking for max performance because:
1. The 2.93 XBench tests I've seen that came in over 220 all have disk numbers that are in excess of the SATA 1.5's (SATA I) 150 MBps speed limit (for example one 2.93 with an X25-M did an uncached read of 256K blocks at 203.85 MB/secs). Unles the UMBP are SATA 3.0 Gbps (SATA II), those tests are using striped raids.
2. Also, the X25-Ms long-term performance has been questioned, but my X25-E has been consistently lightning fast -- I clocked my boot at 23-24 seconds (from power button to desktop). Not only do apps open instantly, they run lightening fast. Which leads me to...
3. IOPS, IOPS, IOPS and write speed.
4. It does not violate Apple's NDA to tell you that the next OS X will be lighter weight than you think and a 32GB boot drive will be more than sufficient (I have two different boot partitions on 1 X25-E); and (b) the speed, at least with the X25-E (which, as I have said, is already stupid fast) is nothing short of astounding...
5. Oh, and BTW, tests with speeds in exces of 600 MBps are using ZFS file systems. I stopped using standard testing methods on ZFS after XBench showed a disk reading 256K blocks, randomly at 2914 MBps.