I find Greninja a little too broken for OU (alongside with, at least, other three pokemons) for 2 reasons:
(1) Protean Greninja is good offensively because punches holes into opposite teams with relative ease because of the STAB multiplier on every move. Having good stats and ability + good movepool from 122 base Spe is huge. There is literally no reason to not run this monster (except in stall teams, of course). If you predict the switch-in of a check, you can simply U-turn and keeping the momentum with a fast U-turn.
(2) Its newfound ability, Battle Bond, is another viable option if you want to use this thing as a late game cleaner (= when opponent's pokemons don't have 100% HP left). Its ability is really simple to trigger and, basically, you have a huge boost to your Atk and SpA at the same time underlined by LO (in this case the damage output really goes out of control). Basically you have another Mega in your team (because every Megastone boost stats by 100 points, Ash-Greninja gets + 110, albeit until you switch out). Not only that, but you have a Spe boost to clean the opposite team even more easily (from 122 to 132, outspeeding Tapu Koko).
So yes, Greninja is still broken and now as even a dedicated cleaning set with Parental Bond which boosts its Spe by 10 points, thus making harder to revenge kill while still keeping the offensive bonus of Protean.
A simple calc to show the power difference between the two forms:
252 SpA Life Orb Greninja Hydro Pump vs. 0 HP / 0 SpD Xurkitree: 265-312 (86.3 - 101.6%) -- 12.5% chance to OHKO
252 SpA Life Orb Greninja Dark Pulse vs. 0 HP / 0 SpD Xurkitree: 192-227 (62.5 - 73.9%) -- guaranteed 2HKO
252 SpA Life Orb Protean Greninja Ice Beam vs. 0 HP / 0 SpD Xurkitree: 216-255 (70.3 - 83%) -- guaranteed 2HKO
252 SpA Life Orb Ash-Greninja Hydro Pump vs. 0 HP / 0 SpD Xurkitree: 351-413 (114.3 - 134.5%) -- guaranteed OHKO
252 SpA Life Orb Ash-Greninja Ice Beam vs. 0 HP / 0 SpD Xurkitree: 191-226 (62.2 - 73.6%) -- guaranteed 2HKO
252 SpA Life Orb Ash-Greninja Dark Pulse vs. 0 HP / 0 SpD Xurkitree: 253-300 (82.4 - 97.7%) -- guaranteed 2HKO
Keep in mind that with base 132 Spe Timid Ash-Greninja + 248 EVs outspeeds and kills Scarfed Timid Xurkitree with one hit. Ash Greninja hits 20% harder than its original counterpart with Water/Dark STABs, whereas loses 8% of power when using coverage moves.