Hey UK Mate,
I've been attemptinting to calculate the CR of the Grell writeup in Lords of Madness, and was unsure how to calculate the value of a couple of their abilities. Specifically Expert Grappler, their variant paralysis ability, and the sightless ability were troublesome factors. Specifics are below. Normally I'd be satisfied with some educated guesses, but I plan to make heavy use of Grells in an upcoming campaign arc.
Expert Grappler (Ex) : Second tier in Improved Grab ability chain. A grell can take a -10 penalty, grappling witha singular tentacle and remain on ungrappled, rather than the -20 penalty offered by Improved Grab. Their +10 racial bonus to grapple checks is included in this ability suite. I figured that this ability sans the grapple bonus was worth a feat, but I couldn't calculate what the grapple bonus was worth for the life of me. I also noticed that the Grappl Bonus/ Penalty was not figured in the size values you gave in challenging Clhallenge Ratings v5.
Paralysis (Ex) : Functions as normal paralysis, but it requires a solid hit. Starts at DC 10. Multiple hits within the same round do not force multiple saves, but instead increase the DC by 1 for every hit beyond the initial strike. Save is Constitution based with a -4 racial penalty. I priced this ability slightly lower then a Ghoul's touch paralysis attack, but I'm unsure on this one.
Sightless (Ex) : Immune to gaze attacks, visual effects, illusions, and other attack forms that rely on sight. I figured that this ability was about on par with immunity to mind effects.
I'm afraid that I might have over-rated these abilities. While my results of a Silver Rule CR 8 pan out with what I expected to be an under rated creature, I believe I rated the Grell a tad too strongly.