XR8 vs. RS4
I got the PRP XR8 and currently I have the factory set plus a spare F/R set from the Serpent 950R. I saw your question and sat down to compare it with my Mini RS4 (the same as HPI RS4) and the shocks are totally different.
It may seem first hand the 1/8 are the same or smaller, but its not. The rear, be it XR8 or Serpent, have longer shafts compared to the HPI RS4 shocks.
The front are also a little longer than the regular HPi design.
The shock housing and the springs width I think are almost the same, but still too long so it will be a tight fit for my 1/10.
I hope this helps.