Actually an idea had came to my mind...Can the spring fail? I know when I compress the rear spring it is so much easier than the front ones (I know they are softer but that was a whole lot easier), and I always feel the back end of my car sits lower than the front. But, how can a spring 'fail'? and how can it fail on both side together?

Anything can fail. It is a little odd to think 2 of them would fail at the same time, but one could have failed a while back and then the other.

I just compare the spring free length, lower inside diameter, and upper inside diameter with a post that another forum member posted with new spring measurement and it's identical...I hope that means the spring is good. but then the metal can fatigue and stuff...this is really hard to find out without some big money...

I wonder if all rx8 sags a bit in the back like mine...

I think I found why...after 3 hours of searching...
I have read threads that people has driver rear sag more than passenger rear, and they swap springs and the sagging remain the same, so the issue is not spring in that case. I also read another post that people swapped out OEM shocks with Koni and the sagging remains...so the issue is not shock in that case...
and then I came upon this article saying that Mazda design MX5 and RX8 so it rides ON the bump stop. In fact the bump stop is part of the suspension and it aids the spring rate as it's being compressed. Since the bump stop is pretty much gone in my car it is very likely this is the reason for the sagging. I will confirm this when I install the new bump stop next week. Interesting no one has brought it up on the RX8 forum and I found the info in the Miata forum:
http://forum.miata.net/vb/showthread.php?t=401320
I learned a lot today reading....now I feel content and ready to go to bed. :P

ok...after the new bump stops were put in...the rear still sags! I give up. I guess that's just how the car is...anyway the koni's are in, along with the new bump stops, the car feels really solid. I can't wait to autocross it. :)