I had to go reread the 11 issues to figure out why my take was so different from yours. It's not so much that either of us are wrong, but more a matter of the assumptions you make that I don't. Assumptions that have to be made, because the writer leaves it unclear.
Since Cassandra had access to all of Xavier's experiences and memories, she knows what younger Jean was like before she was bonded to the Phoenix.
When confronted with Jean in X-men Red she faces a Jean sans the Phoenix Force.
For her comparison to make any sense she'd have to compare a younger Jean to the Jean she is facing.
The comparison she makes falls apart if Jean boosted by the Phoenix force is what she was referring to, because Jean+Phoenix is on the level of Galactus, the Celestials, Eternity Etc. If the writer intended that, then the comic would have ended right then and there. Cassandra at her most powerful is nothing compared to the Phoenix force.
I don't think I minimized Jean's powers in X-men red. I just noticed how much she had to depend on her teammates to beat Cassandra in their battles. You don't seem to acknowledge how critical they were to her winning or that Jean wearing Cerebro.
Cerebro dramatically boosts a telepath's powers. Jean was wearing Cerebro when she confronted Cassandra. Cassandra had no such boost or help in any of their battles.
In the first confrontation while Jean was wearing Cerebro, Jean was unable to keep Cassandra from getting into Cerebro despite being 1000s of miles away. Jean only pulled out a victory thanks to Storm launching a massive tidal wave that temporarily disrupted Cassandra's efforts.
That led to the next battle involving Rachael.
To describe it as Jean vs. Cassandra isn't quite right. Jean was wearing Cerebro and this time wearing it for battle (the writer implies she overclocked Cerebro / hardened it against another intrusion
)
Jean says the only reason she was able to free Rachael was thanks to Rachael fighting hard against Cassandra's mind control. After that fight ended, Jean said she didn't appear in person because she isn't an idiot implying she would not beat Cassandra in person.
Her ability to fool Cassandra wasn't due to Jean's power alone. It was due to the boost she was getting from Cerebro and the assist she was getting from Rachael as she resisted Cassandra's control. I don't think the writer has to spell out that Cassandra was handicapped by the need to dedicate a lot of her power to keeping Rachael, a formidable telepath/telekinetic, under control.
When Cassandra said she underestimated Jean she was referring to Jean's intelligence. She was supremely confident that she was so much more powerful than Jean that there was nothing Jean could do to stop her.
She consistently underestimated Jean's resourcefulness in countering Cassandra's absolute advantage in terms of mutant power. Jean's victory was thanks to a good strategy, and not a result of Jean being more powerful.
Generally speaking I'd agree with you that Jean is the world's most powerful or one of the top 3 most powerful telepaths.
In this comic however the Writer throughout made it clear Cassandra was on another, higher level, and nothing makes this more clear than the final battle.
You point out that she was able to resist Cassandra's effort to destroy her mind, but you forget that Cassandra was not just facing Jean she was also facing The Avengers (including Thor and Ms. Marvel), Namor + the military of Atlantis and Mutants as far as the eye could see while keeping thousands of humans under her mind control. With so many distractions, it shouldn't be a surprise that Jean could resist Cassandra's efforts.