Over 250' is where my Z Buzzzes excel for me, they're very forgiving and predictable. I haven't thrown a FLX Buzzz, so I'm not saying your wrong, but it's hard for me to imagine getting more predictability out of a FLX disc than Z, especially in a mold like the Buzzz. Maybe you're talking better range control with the FLX?
EDIT: After reading your post again, maybe you're talking about trusting your Z to go more than 250'. If that's the case, I'm guessing that your increased distance with the FLX has more to do with the lines you're throwing due to the FLX's wear (like you said) than it does with the comparative glide of the discs.
It must be the Z Buzzz I have and or the trust I have in it. I used to use the Z for anything 250' and under. It never drove well but was fantastic for anything that required finesse, especially anything that required a soft hyzer. I even used the Z for putting. Whenever I get into a selling mood and offer up almost everything I own so I can go nuts and buy new things, I will never part with this Z Buzzz because of how reliable it was in my short game.
However, I have switched over to using a Drone and Meteor mix for my under 300' short game and I have found they do about 99% of my shots in this range.
I now use my FLX Buzzz for soft, slow drives that require a ton of control (especially slow bootlegs to the right) and for control tomahawks at 180-240'. I think this is where all my comments about FLX come from. It is a matter of trust and that is what affects my game. The rubbery feel of the plastic gives me a more confident grip, I feel that I will get a more consistent release from the plastic and thus can put more power and snap into it. Time and again, and blame it on my grip, I have experienced early releases from Elite Z and Champion plastic because of how slick they feel and thus on occasion I have over-compensated and get a horrible case of the griplocks. I have put my Boss, Riot, Z Surge, and Striker out to 400 feet a few times but my most consistent disc over the last two years has been a FLX Surge that time and again has been on target at 340'.