Well, the new Adaptec 5805 RAID controller is now in place and, after flashing the SSD firmware up to the latest revision and doing a secure erase, it now has charge of both my SSD, which is configured as the boot disk, and the original four disks configured once again as a RAID 10 array. The motherboard SATA controller has been relegated to optical drive duties and the motherboard SAS controller has been disabled.
Doing a fresh installation of Windows 7 x64 has totally refreshed the machine with an interval from the end of BIOS initialisation to the Windows Log On screen of just 16 seconds.
With hindsight I've discovered that by hanging the SSD off the Adaptec 5805 I've lost the ability of Windows 7 to recognise the SSD as an SSD so the W7 optimisations to treat such drives kindly are not available, at least not with the current Adaptec firmware. The trade-off is that I can migrate both the Adaptec controller and all of the attached disk drives to a new motherboard in the fullness of time and not lose a single byte of data in the process.
IMHO the difference in disk performance, fully described elsewhere
, justifies the cost of the controller, roughly equivalent to a moderately high-end Intel CPU.