Macintosh bootable volumes are supported on the internal 2-port PCI card (part number TSATA) but are not supported on any of the other SATA cards at this time.
Yes, this is a great way to expand capacity and performance. If two or more Tempo-X, X4+4, X8, X4i, X4P, E4i, or E4P cards are installed to the same machine they must all have the same firmware version to avoid conflicts.