I've found a lot of questions on .inputrc, and the answers included people using Mac OS X terminal.app.
But I've set this property on and it didn't change readline's behaviour. I tried the same .inputrc on Ubuntu and it worked…
Is there some specific action I should take on terminal.app?
Well, I think I've finally cracked it. I will answer my own question for future reference here:
Mac OS X uses editline (descendant from libedit that kinda emulates readline) instead of the more commom readline command line text handler.
I'm still learning how editline works, but I guess it isn't 100% compatible with readline's .inputrc.
So my completion-prefix-display-length attribute probably doesn't work with my version of OS X editline.