Hello, are there any real advantages to be "pillar of society" or is it nearly the same as having an admire standing of 999?

Not noticeably. Possibly a better discount / trade price at stations and maybe a little more faction loss if someone kills you in that factions monitored space, but not enough to be really noticeable.

You used to be able to get all 13 POS which was kind of an achievement / badge of honor type of thing. The game has improved in this area though so it is no longer possible.

No extra faction loss, no extra pricing (the pricing myth comes from the reward for 5 good mentor points). No extra anything in fact. The only reason to go above +600 in any faction space is simply to access some of the higher tier equipment. For example Corvus require +800 for the Corvus Marauder.

Bragging rights for all 13 used to be a reason, but even that has gone now. I prefer 12 KOS anyway, its actually just as hard to get these days too!

Really no extra faction loss? If so that's bullshit.