While I doubt they get all those buffs (there's probably a catch) and they're probably obscenely expensive, I do have to question why the Dark Angels seem to have more elite units with WS5/BS5/W2 than the Grey Knights. For all the Ward-hate, GKs have always explicitly been the best of the best most elite Marines in all the Imperium. And apparently every other SM Chapter is a bunch of lazy couch potatoes.
And, yeah, the landspeeders are terrible. They could have been pretty good, but no. They're terrible. I would convert from a normal Land Speeder, stick some plasticard and extra guns on it.