Hello. I recently purchased this modification and I want to share how I feel about it.
Pro: 1) Easy, quick installation(step-by-step guide would've been appreciated, however)
2) It adds reasonable amount of stability and I could see noticeable improvement on ground consistency!
Con: 1) The shaft is now stuck and you can't pull it out unless you pull the screw and separate female(?) grinder. This makes cleaning more difficult than before.
2) Added horizontal beam hinders flow of beans. I found 2 and 1 beans stuck at the mod each time I used.
Side Note: It's not exactly a con, since its Hario's fault, but this mod does not completely fix the problem. Even though bottom part of shaft is stabilized thanks to this mod, there is still misalignment between both grinders because the top part(the whole plastic bowl thing Hario made) and its hole is misaligned from the start. I don't know if it's matter of QC, nonetheless the problem lies with Hario and can't be fixed by the mod.
I think this is mod is great, get you noticeable improvement right away after installing it. Worth the price. Just don't have your hope too high, thinking it will turn Hario's grinder toe to toe with grinders several hundreds dollars more expensive.