252+ SpA Choice Specs Tapu Lele Thunderbolt vs. 248 HP / 252+ SpD Celesteela: 230-272 (57.9 - 68.5%) -- guaranteed 2HKO
252+ SpA Choice Specs Tapu Lele Thunderbolt vs. 248 HP / 0 SpD Assault Vest Celesteela: 214-254 (53.9 - 63.9%) -- guaranteed 2HKO
The bulk difference is really small and having a 75 non-STAB power move as your only form of recovery will make you die really fast.
If you want to deal with bulky water types there are better options for that, and most of them make great partners for Celesteela.
If you're going to run that set anyways I would recommend a Quiet Nature instead of Modest so you can still use Heavy Slam effectively, but please dont, your set is trash.
unpredictable =/= good