I personally like the Tendril rules as written. It's simple, easy to remember, and effective at relaying the text of the ability. I think that adding bonuses to it's CC, which is already respectable at 13, and a bonues to DEF, which stands at zero, would need it to be increased in points somewhat, as that would seem to make it a bit undercosted, and as previous posters mentioned it would bring up another issue with Feint. So I think the +4/-4 to the opponents DEF/CC is the better option, as it reflects the opponent getting worse, as opposed to the Golem getting better.
And, as the Absorption goes, as I read it, the Golem would absorb the original wounds of the model, so it would absorb all wounds of a multi-wound model. Powerful, yes, but not overly so, and it does have a bit of a precedent in Soul Consumption, as I recall.
Another thing that may be worth mentioning is whether or not the Golem, if it splits, can continue to split, and if the Golems that split off can themselves also split.