I have a 1979 i28. We are almost complete with that and additional structural repairs.
The Compression post may shrink a bit, ours was about a 1/16" to 3/32 shorter than the bulkhead it is attached to. The real problem is too much load on an improper material, i.e. the 3/4" plywood cabin sole.
On our boat the post loads down on top of the plywood cabin sole and crushes it, we had a bit more than 3/8" squeezed out of the sole and a slight deformation on the tops of the floors the sole rides on. I believe it totaled about 1/2" "crushed out".
On the i28 I race on, they had installed a plate as a repair. When they adjust backstay tension they still pull until the head door almost jams...
First off, how about a more details regarding your boat, like do you have a teak cabin sole or just the plywood and carpet...?
Thanks for the reply Phil. My boat is a 11979 I28 as well. The sole is plywood, and the problem is identical to what you described. We do not have crane or dry dock service back yet so I'm trying to address this issue in the water. I can't get the jack directly centered under the mast, but I think it will be ok.