Well - lets see if I can help. Yes, the Prolific PL-3507C is the chip set to use, but it's both Firewire and USB2. With Firewire you can get to 16X - with USB2 you can get to 12X.
I know you stated your case has the Prolific ChipSet but only USB2, therefore it will not be the PL-3507C.
Now here the problem. If I remember correctly many messages on this board state that the 1625 can do 16X but can't do 12X (16X or 8X only).
Will all the above I think the best you will get is 8X burns. Hope another member can give you more details...