If you still use the AppleAHCI class it makes no difference the device id (the Generic AHCI matches all controllers anyway) or the controller name (its probably only used for identification purposes).
I have no noticeable problems (just shortly tested) with ICH10 on a P45 in 32 bits, maybe Apple is doing something weird to the controller in 64 bits.
Just a wild guess, but maybe Apple is doing different things on same AHCI controllers depending on the computer class, try changing the computer identification (MacPro, iMac, MacBook, etc.) in smbios.

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AsRock P45TS | C2D E8200 | GeForce 8600GTS
Mac OS X Snow Leopard 10.6.1 + Windows 7 Ultimate 6.1.7600 + Fedora 11