I`ve got a Oly K9GL with a 16' barrel and I`d like to cut it down and was wondering if there was a optimum barrel length for the 9mm bullet?

Does it matter? 10',11' or 12'

My understanding is that a longer barrel will possibly make subsonic ammo supersonic...  due to higher velocities of course.  Most suppressed barrels seem to be in the 5-9" range.  I have a 7.25" ADCO barrel and it's plenty accurate out to 50-75 yards and very quiet with the can on.  If you aren't going to run a suppressor then sure those lengths would be OK.  I like a compact PDW style 9mm AR, but that's just me.  

I plan on going 10.5" on mine mostly because I like the look with the front sight tower.

It really is a personal preference and depends on your application.

I went with 8.5 for a number of reasons.  
It is even with the front of a standard carbine 4 rail, balances very well, and will still be under 16” with a suppressor (depending on model).  On the other hand I paid to have it short I wanted to go as short as possible but keep it useful without a suppressor.