I think Quinn is great except for a clunky kit. To actually boost stats on her I feel would make her op rather than more balanced. She feels balanced (However, that means she is underpowered against meta picks ^^) and some of her kit feels like it could be better, most people don't like her ult. My only problem is the movement speed falloff and the low dmg "execute" but even as an adc going in on people you don't necessarily want, but for a guy who plays too safe like me (even in lost games I die around 6 times at worst. If you die more than 10 times, that's way too much.) It helps to get in.
Anyway, mostly it's her passive being clunky; choosing to mark as soon as you start attacking but giving you no damage boost, marking a dumb target, not marking the guy you are trying to kill unless he is really far away, etc. Then her autoattack is just a bit off from other champs for some reason and doesn't make sense, unless you only play quinn for a while it can result in your opponent walking away with a harrier mark on them because you canceled your auto when you q'd even though it looked like it hit.
Otherwise I'd say she rocks and if this stuff got changed she would be probably not op but very reliable as she is already pretty good. Short range is fair (and it's not much shorter anyway) because of her burst in trades I think, harrier mark atk speed is great, it would help if q procced harrier and long ult cd should either be shortened or give boosts on abilities because it's good and that might make it op but it is hit or miss sometimes.