Been meaning to ask.

I noticed when I get to about 80mph, my steering gets super light and extremely easy to the touch.
Anything before 80mph seems very normal.
Is this a characteristic of the car?

 

I think it's called aero lift, these cars are shaped like a wing!

 

LOL...To be honest, that's exactly what it feels like .
Feels as if the front of the car gets extremely light.

 

When was the last time you had a wheel alignment done?

 

Given that the XK8 has speed sensitive power steering you might want to eliminate that as a cause.

I would remove the appropriate fuse to disable the system and see if that makes a difference.

 

Just did about 6 weeks ago:

New Bilsteins and mounts
Tie rods
Wheel alignment
New General G-Max AS03

Front end is very tight

 

Do you have the under engine plastic liner (not sure exact name), if missing air could be getting fed up into the eng compartment and causing disturbance and lift. I've had numerous runs well into triple digits (20-30 mile hops) and car felt like it was on rails.