The HP MultibayII drive option I've seen on EBAY are 80GB PATA drives.
The optical bay systemboard JAE 50 connector is simply a secondary PATA IDE port. A SATA-PATA caddy needs a bridge chip to convert from one interface to the other. A PATA-PATA caddy does nothing other than rewire from 40-pin IDE to JAE50. So given there are fast PATA drives doing ~60MB/s, I would *assume* a PATA solution would likely see a better result than your 15mb/s.
I'd be double checking things like UDMA-2 is enabled against the device too. HP might be able to advise on what sort of performance their PATA caddy would give. Otherwise, perhaps ask newmodeus for advice. Better results with the SATA device would be seen if you too could identify a hidden native SATA port connector on the systemboard and wire it directly to your drive instead, as described above for the HP 2510P. ICH8-M provides 3 SATA ports..
8710W also has an unused 24-pin socket on the systemboard as can be seen at the
HP Media Services Library which *speculation mode on* could have SATA I/O pins on it. Then again, that socket could be used for diagnostic purposes as well.
There is one further variable here. What is the chipset that you are using? ICH8-M on a 2510P has a PATA controller onboard provided with it's southbridge. Google tells me 8710W uses PM965, so it should also be using ICH8-M. I know that Intel was planning to ditch the PATA interface and if so, then the native ICH controller is SATA and HP would have had to incorporate a SATA-PATA bridge on the systemboard to run legacy PATA devices like optical drives.
Oh.. can you also identify the SATA-to-PATA bridge chip on your caddy? It will be the biggest chip on there.. Marvell, Sunplus, Acard, Jmicron. newmodeus were tight-lipped about what they are using in their up coming 9.5mm caddy.