I checked my extensions under system report and 30.107 and CUDA are loaded. Uninstalled and tried a patched *25.106. *25.106 This lets Cinema 4D use hardware acceleration. Octane and Geekbench still can't use CUDA. I'm pretty close to reverting back to Sierra.
I saw before that you use the system definition of iMac13,2. That could be an issue. I don't follow use of Mac Pro SMBIOS under High Sierra, particularly with older chipsets, but I was surprised you used an iMac definition with dual cards, since Apple has not had or allowed dual GPU cards in their own hardware except for the Mac Pro. Another consideration: with High Sierra, the recommendation is to put all added kexts into ~/EFI/EFI/Clover/Kexts/Other, I believe. I have no added kexts in S/L/E or L/E, am glad for that. I still use HFS+, for now. I keep the disk EFI folder matched, but I use the
Clover bootloader from my "installer" USB to start up, and the config plist in the USB, at least, configures the startup. That saved me trouble adjusting to Sierra/High Sierra.
If for some reason you aren't with your system definition, be sure to use the latest IntelGraphicsFixup kext
https://github.com/lvs1974/IntelGraphicsFixup and the latest Lilu kext
https://github.com/vit9696/Lilu/releases.
Before you revert to Sierra, I'd try different system definitions. I suggest having a newly made USB installer [on a different USB flashdrive] made either with UniBeast and then updated with your config plist settings, kexts and drivers or the same using RehabMan's install Clover to USB method:
https://www.tonymacx86.com/threads/guide-booting-the-os-x-installer-on-laptops-with-clover.148093/. Normally you should have a filled out SMBIOS in Clover Configurator using P1LGRIM's method for a fully usable SN:
https://www.tonymacx86.com/threads/an-idiots-guide-to-imessage.196827/, but to test different system definitions you can keep the SMBIOS simple by choosing a definition and a random SN--and make sure you
select Trust (and uncheck Mobile), which too many people miss. Again, the USB EFI/Clover should have your modified config plist and your drivers, and your latest kexts, including NvidiaGraphicsFixup and Lilu, in Other. Just the last couple 10.13 updates have caused some people to change their system definitions. Try MacPro and iMac definitions, but I wouldn't get hung up on matching your chipset/CPU because of the dual cards. It's about finding what Apple allows natively and modifying it if necessary.
Definitely keep a clone of your High Sierra install no matter what, because new drivers will be coming out. But keep trying in High Sierra. apple pic, for example, though using Z170, has dual 980 Ti's running on 10.13.4:
It seems like 30.106 is the most functional for those who are using one GPU (most people), but for the few of us that are running dual GPUs, 25.106 is the closest thing to fully functional on High Sierra.