Buu saga Vegito isn't a good gauge because we don't know how strong he was. We just know he was much stronger than Buuhan.
When SSG was introduced, we didn't know how strong it was compared to Buuhan. We just know it was much stronger than SSJ3 because of how Beerus treated Goku in those two forms.
A better gauge would be Merged Zamasu. He appeared last arc and we saw how he fared against SS Blue. Merged Zamasu is made up of normal Zamasu and Goku Black.
Normal Zamasu is around SSJ2 level, probably a bit weaker.
SS Rose Goku Black = SS Blue Goku (maybe a bit stronger)
Caulifla is SSJ and as we saw a week ago, Kale is t as strong as SSG Goku since Goku was fighting both of them as SSG without a problem.
That means Merged Zamasu is much stronger than Kefla even if we factor out the immortality.
And remember, Goku actually did quite a bit of damage to Merged Zamasu with a fully powered Kamehameha as SS Blue.
If he can damage Merged Zamasu as SS Blue, he should be able to handle Kefla. Using Kaioken with SS Blue means he should be rag-rolling Kefla.