I understand you considering a brain swap. I do hope however not to put your expectations too high here. First look at the CPU performance during Lightroom, I don't think it's hitting 100% for a considerable amount of time. If performance is low then the CPU is waiting for other processes or peripherals. Here a faster CPU would not solve anything. As with your GTX1080, this one should fly, also with 2 monitors attached and 8Gb video memory.
I suspect Lightroom doing something on your mechanical drive. I sometimes see weird things going on with Photoshop (I don’t use Lightroom), building up a scratch file while there is still more than sufficient unused RAM available, communicating to the HDD in 4K datablocks, etc. Please be aware that HDD performance significantly drops, becoming terribly slow, when dealing with 4K data blocks.
As long as you haven’t found the root cause of Lightroom’s sluggishness I’d be a bit careful spending money and efforts on exchanging parts that supposed to be already fast in the first place. As for now my suspicion goes into the direction of the mechanical drive.
Regards,
Jaap