I don't really understand your problems with Kittycat and his kit..
Ofc a fed Rengo is a bad thing for every Adc but wait, what happened that he is fed? Right someone of your team fed him!
Should we nerf Zed, Kha zix, Fizz and so on too, just because they can "one shot" a carry when they are fed?
Rengars Ult is part of his kit and even the stealth isn't really a so big problem.
Why don't they play him everytime in the LCS when he's so OP? Because experienced players can counter him easily, and with the increased range of the "!" which is so terrifying for you, a Rengar can't even Flash jump, that you can't see the "!".
To counter a Rengar is pretty easy, but I'm still playing him often, because only in 1 of 10 games the enemy team just even try to counter him.
And for the Adcmains(I don't really want to say how to counter, cause it would ruin my fun with Rengar) here are just a few thinks what you should think about:
Why do you think, the bloodthirster has a shield, deathsdance has bleeding dmg, I just heard pink wards just look beautifull and red trinkets are at lvl9 also very amazing, why attackspeed shoes when you could(! I told you if you COULD !) skip these[Q is an aa and I thought there were shoes which are a wonder vs aa based champs(10% less dmg!!!)], and last but not least, why are there adcs with dodge skills?
Even when your an Adc, you just have 1(!!) job when you see the "!". Surrive the first impact and you are almost safe, and if your not, where is your support/team?
If a Rengar ISN'T fed, he needs much more time to get his items, so if you see they have a Rengar in their team, maybe stay safe? Not alone? Dont push?
And if your playing adc and he's fed...go to the next corner of your room, stay there with your face towards the wall and say 5 times: He told me to not feed him. You don't need to watch your game, i don't think you will play really much and hey, a grey screen is also something beautifull ;)