Under /ACPI/DSDT, I added:
<key>Fixes</key>
<dict>
<key>FIX_INTELGFX_100000</key>
<true/>
<key>FixDisplay_0100</key>
<true/>
<key>NewWay_80000000</key>
<true/></dict>
The /Graphics dictionary looks like this:
<key>Graphics</key>
<dict>
<key>Inject</key>
<dict>
<key>ATI</key>
<false/>
<key>Intel</key>
<true/>
<key>NVidia</key>
<false/></dict>
<key>NvidiaSingle</key>
<false/>
<key>ig-platform-id</key>
<string>0D220003</string></dict>
Just in case it matters, my /RtVariables/BooterConfig is set to 0x28, my /RtVariables/CsrActiveConfig is set to 0x67, and I've got my SMBIOS setup as iMac14,2. As it stands right now I have both screens working fully at 3440x1440 with full acceleration and have no issues with Xcode, Reveal, or iBooks. Hope this helps.