This is what I know...
The published data about the S-54 Rdstr suspension claims that it has the same calibration as the (then existing) M Coupe.
That is to say, that the springs and shocks are firmer.
That I believe to be true.
As for the ride height, it was most assuredly not lowered. Not the one I have anyway!
When parked next to the 99, it sat (past tense...) at an inch or more higher.
Last winter I installed double adjustable Konis on the 01. Instead of cutting open the 01's front struts (with 3000 miles on them), I swapped the parts from the 01 to the 99 (with 34,000 miles), and used the 99's struts for the Koni inserts. Nice that both cars wound up getting improved suspension out of the deal.