Rengar main here, I have almost 500k master point on the champions, all the skins, MR7, all the chromas. Love the champ and I love it when he's strong, but he doesn't need this buff. Rengar deserves to be behind in damage early and a slow start, if not, he's a disgusting monster with zero counter play unless you are playing his counter champion, and even then he can 100-0 with little effort if you're off guard. Buffing his ultimate will allow him to be more of a menace early game increasing his snowballing power and adding pressure onto a lane merely by being in the game as you never know when he may come, even with wards.
Not to mention how much longer the ability lasts, 8 seconds longer at rank 3, 28 seconds in total he can be invisible. I personally agree with this one as I believe rengar is a one chance one shot champion, it sucks to ult and just have to walk away because there's no opening yet. Which is the hardest hurdle for a Rengar main to overcome, ulting and deciding that it's best to not take the leap. Regardless, I would prefer if they didn't add this either though, Rengar is in a decent enough state, he's strong, but not god tier, any little buff could put him over the edge and ruin his good faith. Resulting in all the Rengar one shot posts for a month in till Riot caves and kills off the champion with several nerf patches as they did back in season 5.
Any Rengar mains that try to defend this buff have little foresight as I doubt any of us wish for our beloved bush-pounce kitty to get gutted again.