Is there a standard factory deviation from a 90% perpendicular angle when the car is on a level surface with no additional weight inside the vehicle? It seems like my rear wheels are becoming less and less perpendicular, like I have a couple offensive linemen in the back seat.
Or is this simply what happens to suspensions after 8 years of spirited driving?

 

The correct term you are searching for is camber. Alignments will change slowly over time, especially with aggressive driving. If it has been over 2 years since your last alignment, you may want to get one done. The camber will slowly increase (negatively) over time if your cam bolts are not fully tightened. Another possibility is that your springs are sagging, causing the increase in negative camber.

I have attached the factory rear alignment settings for both the standard and sport suspension.

 

Thanks JC. Good answer, I can't remember the last alignment I had done. Never crossed my mind since the wheel doesn't seem to pull to the left or the right when driving.

 

No problem. Many don't realize that alignments are extremely important, especially on performance cars. Most just think that if the car tracks straight, the alignment is good. All this would mean is that the car tracks straight. There could still be too much or too little toe, camber, or caster. You will be surprised what a "good" alignment will do for the RX8.