OS X software RAID 0 with 6x SATA SSD?
I suspect that 6 SSDs can provide more bandwidth than the SouthBridge Controller on the MacPro can support (maybe 8Gbit/s or 1GB/s)? This may also depend on the model of MacPro that you're using.

The throughput supported by the drives themselves may also come into play...Some SSDs achieve reads of about 250MB/s, while others support rates around 480-500MB/s. Your 3Gb/s SATA ports will probably top out at 375MB/s

My money's on a benchmark of about 1GB/sec for your 6xSSD RAID-0 volume's read rates...Probably a little less for writes...You can probably attain the same speed by striping 3 or 4 drives together.

You can definitely install OS X on this striped volume, but you may not be able to put the recovery partition onto that drive.

Regardless, the AJA Kona System Test is a good utility to use for this benchmarking. I'd be interested to hear what you find.