So, it was released a guide with the blu ray version of the One Punch Man anime, called One Punch Man Compus, which features a bunch of stuff from One and Murata. The cover and an example of pages can seen below.
Now jumping to what we truly want, some of the pages are dedicated to the character Lord boros, the strongest Saitama faced thus far, and the article shows what are Boros full capabilities:
Boros is the leader of the Dark Matter, the pirate band that controls outer space [...] Boros himself boasts abnormally superior physical attributes even for his species, along with latent energy so powerful he can blast away entire planets.
So yeah, Boros is a planet buster at his restained form, and things escalated more an gets interesting.
When released, this speed and power goes beyong the phisical limits of what any living being can endure.
So the reason on why Boros gets so strong is because he overcomes his normal limits. This is probably why Saitama is so strong as well. So this shwos that Boros himself is far beyond the limitation of normal beings, oh boy, the S class would be screwed.
To counter Boro`s ultimate "Collapsing Star Roaring Cannon, which is formidable enough to obilerate a star, saitama resorts to his "Serious punch."
So yeah, Boros Collapsing Star Roaring Cannon really was a collapsing Star Roaring Cannon. Boros seems to be a beast.