IMHO going to the 164 makes me wonder, to @Tony S's point, why the MX89...just because it is a Kästle?
If you came in to me on a ski wall I would simply say "I will let you buy it, but I am not going to sell it to you."
Didn't wanted to go into the ski selection bit. IMO, the 89 is too wide and doesn't have enough edge grip for eastern groomer.
If I am skiing "pure" groomer and nothing else. I would definitely go shorter on the MX89. Wouldn't be my first choice in ski though.