Furthermore the exchange between Corax and Curze was more than blocking a single a strike, it states that Curze effortlessly lifted his claw upward in defiance of Corax who was with all his might pushing downward to kill Lorgar. So at the very least from that exchange we learned that Curze far outclasses Corax in strength, as he was effortlessly lifting Corax's downward pushing strike of which Corax had the momentum and leverage advantage and still was overpowered.
Bearing in mind Corax had already been fighting for quite a while against lorgar, in addition to the brunt of lorgars possessed space marines.
The foul irony of the moment settled on his shoulders, feeling
grotesquely apt. He couldn’t move his legs. His body was a temple
to nothing but pain. He could barely even see his executioner,
for his psychic efforts had left him quivering with both weakness
and a vision-blurring ache in his mind. A faint outline met his
gaze, the blurred image of scythe-blades raised high.
‘Do it!’ Lorgar screamed at his brother.
The claw fell, and struck opposing metal.
CORAX LOOKED TO meet eyes as black as his, in a face as pale as
his own. His claw strained against a mirroring weapon, both sets
of blades scraping as they ground against each other. One claw
seeking to fall and kill, the other unyielding in its rising defence.
so no, curze does not far outclass corax in strength, indeed, given the context, nothing shows he outclasses him at all.
In addition, corax was hesitant and he tarried in time, so also in speed, therefore also of strength in the blow.