Sorry, it took me so long. Had a few rough days at work.
So, basically what I did: As I told you guys, I was able to install 10.9.4 without major issues and perform the initial boot. From there on, I was thinking that it has to have something to do with my MultiBeast settings. I just thought for myself, that the Bios for example couldn't be the problem anymore.
I installed Mavericks over and over and over... Each time with different setting combinations and so on.
What did the trick in the end, right after the initial boot I did the following (in this order):
1. In Settings->Security I allowed Apps without restrictions. I figured, maybe some Kext is not installing properly with MultiBeast or so...
2. "normal" MultiBeast run with the following settings:
Quick Start > EasyBeast
Drivers > Audio > Realtek ALCxxx > ALC892
Drivers > Audio > Realtek ALCxxx > Intel 9 Series Motherboard Support Drivers > Disk > 3rd Party SATA
Drivers > Disk > TRIM Enabler > 10.9.4 TRIM Patch Drivers > Misc > ElliottForceLegacyRTC
Drivers > Misc > EvOreboot
Drivers > Misc > FakeSMC v6.9.1315
Drivers > Misc > FakeSMC v6.9.1315 Plugins
Drivers > Misc > FakeSMC v6.9.1315 HWMonitor Application Drivers > Misc > NullCPUPowerManagement
Drivers > Misc > USB 3.0 - Universal
Drivers > Network > Realtek > RealtekRTL8111 v1.2.0 Drivers > System > AppleRTC Patch for CMOS Reset Bootloaders > Chimera v3.0.1
Customize > Boot Options > Basic Boot Options
Customize > Boot Options > Hibernate Mode - Desktop Customize > Boot Options > Instant Menu
Customize > Boot Options > Use KernelCache
Customize > System Definitions > Mac Pro > Mac Pro 3,1 Customize > Themes > tonymacx86 remixed
Maybe EasyBeast was crucial, with DSDT free it didn't work for like 20 different tries.
3. After installation through MultiBeast, I "installed" the Intel HD 4600 graphics fix from tonymacx86:
Note that I am using a dedicated graphics card and the internal graphics are disabled in the Bios, which is why I am not sure if this option did the trick.
Audio through the green motherboard jack works (don't know how this thing is called ), but I couldnt test HDMI Audio yet.
System itself works really well, altough I had one freeze when plugging in a USB hard drive. I need to find out what that was.
Good luck, trying yourself...
Man you're the best... I've been looking for hours and hours and have not come across any post that gave the simple solution that you mentioned. So I confirm: Disabling Turbo Boost does work !