I'm not confident in that range. If Kurnoth declared their reroll I would either not charge them or charge them in such a way that they get far fewer models fighting than I do.
Let's have them fight each other.
3 Kurnoth Hunters w/ Greatswords - 180 points - 15 wounds in total.
Regulars - 8 attacks, 5.3 hits, 3.5 hits, 4.8 total
Leader - 4 attacks, 3.3 hits, 2.2 wounds, 3 total
Orruk Brutes, 1 Gore Choppa, 3 Gore Hacka, Boss with Choppa & Smasha- 180 points - 15 wounds in total
Gore choppa - 3 attacks, 2.7 hits, 1.8 wounds, 2.4 total
Two brute choppas - 12 attacks, 10.6 hits, 7.1 wounds, 4.7 total
Boss choppa - 3 attacks, 2.7 hits, 1.8 wounds, 2.4 total
9.5 to brutes and 7.8 to Kurnoth without rerolls. Brutes come to 6.34 if Kurnoth have rerolls.
I can't see even with rerolls how you would conclude they are better by a factor of four. 25% better - only when rerolls are in effect.
There are many other complexities not represented like when each side might lose a model and who would fight first, but no one should ever expect to be wiped by them unless they've given up every advantage.