How much stronger did he get?

Overall power?

@coolcat4: I am not sure I read an exact measurement to the whole Doomed Superman's power levels, but he radiates an intense heat aura like a mature DCYou Doomsday, and he has possibly the ability to withstand Kryptonite, but that potentially can make him 100 percent Doomsday and makes the Superman persona recessive to the character. He also smacked around Cyborg Zor-El, although I am not exactly sure about the power of Zor-El, but he sure looks way weaker than Hank Henshaw pre-flashpoint.

Pretty strong.

Kryptonite didn't kill him or weakened anymore. It only made it almost impossible for him to control the Doomsday dna.

He could generate null-fields and even learned to control them.

He was able to tear through Cyborg Superman like he was nothing. And Cyborg Superman is clearly weaker in the New 52 version, seeing that Kara took him out with little trouble.

He was able to destroy all of Brainiac defenses of both his worldship that made Earth look like a moon, and even Brainiac desperately tried to reason with him. Not stop him, because at that point I believe Brainiac knew Superdoom was beyond stopping. Brainiac only chance was to try and bargain with Kal.

When Lois used her psy powers to transfer all the psionic energy Brainiac had stolen to Superman he had no problem controlling them.

And Superman survived the inside of a black hole. Now if that was because of the Doomsday virus or Superman kryptonian physiology, no one knows.

They didn't even bothered to explain what happened to the virus. Did it become dormant? Did the black hole energy amplified Superman powers to such a degree that his body was able to flush them it out or eliminate it altogether? No one knows! And at this point well never know.

Pretty much best described by what @heavenlydarkdragon said. But the whole story was pretty wierd and didn't make much sense either, especially when I was left wondering if and how Superman actually got cured at all? I almost thought that his later wierd power fluctuations were the result of not being fully cured, but then they pretty much laid it out that all Kryptonians are having some kind of power fluctuations. The plot is a mess with Superman and related characters right now, it really is. But the big contrast between this and what I would argue is the superior Death and also Return of Superman arc, is the fact that Superman wasn't "helped" or infected by Doomsday, he had to get back in the saddle and fight with this guy...

or at least an earlier version of Henshaw that had Kryptonian machinery for his Cybernetic Parts, if there's anything I despise, it is how they made Doomsday in the DCYou, I could rant on longer, but I need to let some air out.