Well, what's canon?
Angel: After the Fall is indisputably canon (#1-17 & 23).
I consider Angel: Aftermath to be blatantly non-canon (#18-22).
I'm not sure about #24-27 as it was written by Brian Lynch, Whedon's appointee as Producer of the Angel canon series, but...
Then there's Fallen Angel Reborn, which puts Illyria roughly on par with a Supergirl, but less durable than I believe her to be.
Anyway, Angel: Only Human is even more dubious. TFAW claims it's written by Joss Whedon, but I'm skeptical. If it is canon, it's our best example of Illyria's powers.
Personally, I tend to believe that Illyria pre-Time Bomb was as strong as she can be in a Human Shell. Wes siphoned off the bulk of her power, but it gradually comes back (i.e.-healing), and either her Shell will adapt to the power better this time, or she'll have to be drained again.
In helLA, all bets were off, different dimension, different set of rules. Look at Nina, she was formidable due to the different supernatural rulebook there. And yes, I'd say Illyria was more powerful there than she even has the potential to be on Earth. She was moving through time, by Centuries, I doubt even in her pure form she had that ability.