In the two comics he was seen in, Superman Prime only appeared on about 7 pages total.
His greatest feat(s) were defeating Solaris and helping to resurrect Lois Lane.
I have not doubt that DC created Supes Prime to be the end-all/be-all in their company, but is that really enough (feat wise) to say that he could beat The Keeper?
And sentry was created to, apparently, be the end all/be all of Marvel.
It seems that the million exploding suns thing is becomming closer and closer to canon. Hell, even CREEL said that he'd never felt ANY kind of power like sentry's before. And Creel's absorbed Uru and asgard's energies themselves.
The Sentry still doesn't have enough feats to have him beat all the top-tiers though, even if that's how Marvel wants to portray him.
superman prime seems to be something of an anomaly that just exudes power from all over the place (powering herald and up level beings) such as superman 1 million who still did amazing things like time travel with next to no energy, leaping planets and so on) gauging superman prime too tough with the lack of feats but lets just say this, superman 1 million and all the other 1 million versions are nothing to superman prime... so judging by their feats, he can do things probably on a universal scale and maybe if they do it, a multiversal scale... maybe he turns up in a marvel/dc crossover and kicks the crap outta galactus to save earth or something? who knows!
That dosen't mean that Superman Prime has the feats to back up such a claim.
Does he have such feats? Every single time a character is first introduced, they always have nothing but high showings.
As I said before, Supes Prime has appeared in 2 comics, and in those 2 comics he was only on about 7 pages total. Even on those 7 pages, he only has 2 feats of any consequence.
A character with a total of 2 feats simply can not be properly gauged in power. And you can't gauge a characters power based on what you think it might be.