Do we want to increase Str and natural armor at various Hit Dice like the hydra? Anything else besides number of attacks and normal changes based on HD?
Why don't we start with just the normal minimal changes due to increased HD and see if we need to add anything to bump the CR? We should probably aim for CR+1 for each extra tentacle/HD.
I think the CRs are still too high for the higher-HD versions. Compare to CR 6 tendriculous and shambling mound and CR 8 treant.
Edit: On second thought, the high number of attacks (and potential blood drain) seems to give it enough damage output to possibly offset the other abilities.