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.
Another manufacturer's previously-installed SATA driver may be accessing a Sonnet SATA card in error causing a drive access slowdown. For example, we have seen Fusion D400QR5 performance drop from 200MB/s to under 50MB/s in such circumstances. The solution is to remove the other manufacturer's SATA driver.