The MIP will swell up as well. Most of us in this area prefer the Associated orings over the MIP. I've never seen the Yokomo shock but my guess the oring has no place to expand to so it places extra pressure on the shock shaft and therefore causes it to bind or create extra friction.
We have this same problem with losi shocks and their cartridges. With the losi shocks you can modify the cartridge to allow for the orings to expand. You maybe able to do the same for the yokomo shocks.
Here is a link to the information some of use put together to modify the Losi cartridge. When doing this we are able to use the orings for a few months besides just a couple of weeks. Maybe you can use the information for a similar fix to your problem.