When Talaan shields Nynaeve, for instance, it is surprise that wins the day, despite both women being equally strong and holding the full amount of the OP. Same with Nyn vs. Moggy, were the shield rammed home when Moggy got momentarily distracted.

And by itself, Graendal being able to shield Aran'gar is no surprise at all. I've seen absolutely no evidence that Aran'gar is stronger than Sammael, yet Moridin says a Graendal vs. Sammael battle could have gone either way. That is enough proof for me that Graendal exceeds Aran'gar.

As for Graendal's angreal, what is critical is if she had it or not. She does mention losing several priceless OP artifacts in Natrin's Barrow. Even if she had it, though, I really doubt it gave her anywhere near 2x her strength. Elayne is a useless counter example since she has no great experience with angreal, and only had stronger angreal to make her comparison with. The weakest in the bunch seemed weak to her, which is no surprise.

But with Graendal, the case is different. In her own words:

The angreal did not offer much, yet anyone who thought they knew her strength would find a shock.

2x Graendal is definitely stronger than even Rand. Saying that wasn't offering much is a stretch, but then she compares what she gets from it to her own strength. The implication is that it makes her somewhat stronger, but not enough to overmatch stronger Foresaken. If she could do so, she would have said she can overwhelm anything they throw at her, which she clearly is not.

Thus, even with the angreal, I find the fact that she wove two shields and a gateway impressive, and confirming the fact that she was stronger than Aran'gar. It may indicate that as Lanfear was to Moridin, Graendal was to Demandred.