The KPS88 is definitely stiffer than the K90 - I own both.The KPS88 didn't bug my arm because I didn't play with it long enough for it to. It was too stiff for me in that it made it too powerful and didn't have the more flexy feel that I like. The heavy weight also made it hard to control and maneuver.
Sampras used the St. Vincents, which were stiffer than the China versions. They probably had a stiffness rating around 66-67. I prefer the TW re-issue version which are very comfortable at only 62 RA. However, it is very low powered due to it high flex and low swingweight.
IMO, the KPS88 felt stiffer to me than the K90.