You can check your Macintosh System Profile. When you select the installed eSATA card, the Device ID will show either "0x6081" or "0x5081". The 3 Gigabit/second eSATA cards use the Marvell chip 88SX6081. The 1.5 Gigabit/second eSATA cards use the Marvell chip 88SX5081.
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.