I've found on the interstate over 75mph the car becomes
very very light and feels like it want to just change directions.

Anyway to improve stability?
I'm at stock height with OEM suspension and considering
maybe Swift or Eibach springs would lower me an inch
and help with this.

And I see PWJDM does sell an underbody carbon fiber piece
to reduce under car air instability but I'm sure that's really
for track cars exceeding 130mph.

 

check your alignment. some toe in will help stop wandering.

 

It shouldn't have done that IMO. I've done in excess that easy with no lost of traction and this is with no aerodynamic mods whatsoever.

My first suspicion is your tires...

Stock S should be able to get over 75mph easy with no drama. TBH, I feel my S more 'planted' to the road at higher speed.

 

Set up your car with the UK alignment. It will feel plant at any speed.

 

Check this link from two weeks ago.... He is asking the same exact question.

High Speed Areodynamics