lowered with coilovers, HR sways and 19" wheels car feels unsettled in high speed corner (over 90-100 mph. like its moving around. not planted and stable.

any ideas what would cause?
could this engine and tranny mounts not being stiff enough to handle all the other suspension tightening?

 

Was an alignment performed after the install? Double check screws, bolts, mounts, etc...

 

Make sure that whoever installed your coilovers that they set the ride height in accordance with the mounting instructions. PSS9/10s, for example, only give you a 10mm (3/8") range that you can set the ride height for optimal performance. As many people on this board have found out, when you are set too low you will be riding on the bump stops which will give you the exact unsettled feeling you describe.

 

Have you done an alignment lately or have new tires? I have stock suspension and last week replaced 2 rear tires followed by an alignment and handled just liked you described. The off-ramp by my house when I drive my car now it feels darty/floating almost twitchy feeling at 83+. Make long story short its at the dealership getting its 3rd alignment to address issue I'm having.

 

alignment was done after install by reputable shop.
could it be the damping setting being too low? i have at almost full soft.

 

Too low and too soft will result in the ride you are describing.