it was said that Sakumo had a reputation as high as the sannin, that doesn't mean he was stronger at all.
out it this way, the title of "Kage" is higher than anything else, including sannin, yet the sannin can take almost every Kage out there.
so no, he wasn't stronger than the sannin, if anything on par with just one sannin. and I doubt Kakashi will surpass the sannin unless he is granted a crazy powerup. yes he will surpass his father though. when kakashi said the next generation surpasses the previous one, he's talking within that same family tree.
your logic is a little flawed.