I'd say that Cell being a melting pot of Goku, Vegeta and Freeza should logically make him able to achieve greater gains than any of them in similar situations, so considering current Vegeta's power ups, Cell ought to be approaching on Hakaishin territory at the very least, if not outright surpassing it. He easily beats everyone up to Super Super #17, where things get a lot more tricky to discern, though with the post-Rosat gains, I think he should be capable of taking it.
As for #21, it's difficult to say. #16 stated that #21 had more potential than Cell, but never mentioned to what degree. Unless #21's training consists of her powering up through eating everything she can find like a hot Galactus, I don't see her being able to win with just 1/4th of the training time.
Assuming the last opponent is made with the Super Dragon Balls, I don't see even this hypothetical Cell doing anything to them. I don't even need to describe how much the difference between a dragon who's never restored more than a planet or a few billion people is compared to a galaxy sized one who's supposedly nigh-omnipotent.
SS4 Goku and Vegeta (Z) VS Evil Boo
- Goku can't use the Ryu-ken and Boo can't absorb anyone or use his candy beam.