Both Whispers caches and Helltide chests will drop higher Item Power rewards starting with Season 2! This change is part of Blizzard's continuous efforts to optimize item progression in Diablo 4, giving players more and better ways to obtain item upgrades.
In their current state, rewards from Helltides and Whispers provide significant upgrades at certain levels or character progression thresholds (for example, right after entering World Tier 4). When players pass these thresholds, running either Helltides or Whispers right now isn't really worth it if you're hunting for meaningful item upgrades.
Starting with Season 2, running these types of content will be much more worthwhile in terms of raw item upgrades, not least thanks to the adjustment of the Item Power cap and its relation to Monster Level.
Additional changes to endgame content like raising the base level of Nightmare Sigils rewarded from Whisper caches or the addition of farmable boss materials dropping in Helltides and Whispers are intended to give players more reasons and long-time motivation to run different types of Diablo 4 endgame content.
Endgame Improvements in Season 2 New Item Power Cap Is 925 In Season 2