Best possible hardware performance?

Is there any hardware specs that would make editing complex mesh lag-free/real time? I have a GTX 1080, SSD, 32GB DDR4, Intel i5 6600k, etc. and I still notice quite a bit of staggering when working on very high detail mesh landscape, generated in ANT landscape. 2Mil or so tris, which really isn’t that many compared to what my computer can easily cut through in zbrush.

Edit: for example, would adding in another gtx 1080 increase my modeling performance (not just rendering) or is there a cap on blenders viewport performance

No there are not. Edit Mesh code needs to be rewritten for updated performance.

What operating system are you running?
I have noticed better performance under Ubuntu 14.04 (x64) than Windows 7. Overall 3d view, like navigation, zoom, switch edit/object - all smoother/faster, in large files where stutter begins on Windows.