I've got my 9400 GT fully working with CI/QE now thanks to tips from Fireofanor, Taz, and most importantly Aqua-Mac who's created an installer with the latest Apple nVidia driver release that updates drivers for the new cards/GPUs. That is the key.
Short guide on how I did it:
1) Download and install Aqua-Mac's nVidia driver installer. Don't think you need to remove exisiting nVidia kexts. Don't reboot yet, though installer will ask you to.
2) Remove NVinject,kext and NVkush.kext if you have them installed.
3) Create EFI/GFX string in com.apple.Boot.plist as suggested in previous posts. I find the easiest way to do this is via OSX86Tools.
a. Use Add EFI Strings/Boot Flag on main menu. Select GFX strings option.
b. Scroll to bottom of list, select Custom nVidia GeForce.
c. For name model enter "NVIDIA GeForce 9400 GT".
d. Select the amount of memory on your card.
e. Select the display output that matches your card. For my PNY Verto, it's dual DVI-I ports.
f. The plist info and hex string for your card will pop up. Select and copy the hex string for safety, but use the "Import String to Boot Editor" option. Click Apply Change on the next screen that pops up, no need to add other flags or strings.
4) Use repair disk permissions and clear kext cache on OSX86Tools main menu or do this any way you like.
5) Check the /Library/Preferences/SystemConfiguration/com.apple.Boot.plist file for safety sake using terminal and nano or vi just to make sure the string was inserted properly. If not, use the version you copied to the clipboard and add the "device-properties" key and value manually. (Sudo to edit file.)
6) Reboot and enjoy the results! No mods to driver kexts with device ids or anything else is required.
My SP output now:
NVIDIA GeForce 9400GT:
Chipset Model: NVIDIA GeForce 9400GT
PCIe Lane Width: x16
VRAM (Total): 512 MB
Vendor: NVIDIA (0x10de)
Device ID: 0x042c
Revision ID: 0x00a1
ROM Revision: NVIDIA GeForce 9400GT OpenGL Engine [EFI]
Resolution: 1440 x 900 @ 60 Hz
Depth: 32-bit Color
Core Image: Hardware Accelerated
Main Display: Yes
Quartz Extreme: Supported
Status: No display connected