The utilization on them cards is so low, Have you tried forcing it into fullscreen, i have this issue on assetto corsa it looks full screen ( i think it's called borderless fullscreen? ) and i get 70fps ish and like 40% card use when i alt + enter i get 99% card use and 144fps :/ worth a shot?
What's it like with a single card? Might be worth trying of you get sick of troubleshooting and just want to play it.
Clean Build
Have you tried Revo Uninstaller? It's very good at locating remaining files once the uninstall process has completed. It may also be worth looking at CCleaner!