Based on the greater number and wide variety of attractions at MK, I think it will end up being by far the least affected by FP+, whether there is tiering or not.
At MK, there are people and groups who would love to ride the mountains multiple times, but have no interest in the character greetings or rides that are meant more for children, and other people and groups who are just the opposite. The total ride capacity per person is a lot greater than at the other parks.
If FP+ are limited to 3 at MK, and are tiered, the total allotment of FPs for most attractions may not even be taken in advance, except at maybe the very busiest times.
If you have been at WDW the last two weeks, or are paying attention to wait times on
MDE, the wait times and FP return times at MK have been low, even for popular attractions. There have been several reports of people riding Space Mountain several times in succession with posted wait times of 10 minutes and actual wait times even less. I witnessed that myself (although I had no desire to take advantage of it) last Wednesday.
For people who have said that they go to MK at less busy times and have been able to ride whatever they want without ever waiting more than 20 minutes, the fact is that you can pretty much do that without FPs at all. All you have to do is get to the park early and move purposefully around the park, making sure to hit the attractions that will eventually get the longest lines (like PPF) earliest.
Because of its greater overall ride capacity, the MK is a prime candidate for some type of same day FP service that will allow guests to supplement their advance FP+ reservations in a manner much like FP-.