That was a result of his trip through the Lifestream, as I said. It took years for those powers to accumulate.
You only assumed that, and if he just happened to gain the same powers Jenova already had it's a pretty huge coincidence.

His transformation wasn't complete, but he was getting there. Once Meteor hit, he would've bonded completely with the Planet, finally becoming a 'true' God.
So he can get to near God status through going for a swim in it, but he decides he wants to destroy the planet just for that extra push?

Hojo knew that Jenova's dismembered body would find a way to reform itself. Jenova was incapacitated, so Sephiroth took charge and summoned the Clones. Manipulating htem, convincing them to find and bring the Black Materia was just another 'bonus' for him.
Hojo knowing that would suggest he knew Jenova could survive while split up. There is no evidence Jenova is incapacitated. It's weakened, but only compared to it's former self, but it's still damn strong. The reunion would restore it fully though, and is far more beneficial to Jenova than Sephiroth, as I can't imagine Jenova letting a human rule anything once she was back to full strength.

Sephiroth was widely acknowledged as 'the greatest warrior of all time'; the fact that his 'damage numbers' are a little lower is merely an issue of game mechanics. In his time, he was the greatest fighter in the world. By the time he got to the Lifestream, he was angry and determined; it's not necessarily irrefutable that Jenova had taken over his mind. After all, he allegedly came to his conclusions about his 'rightful place' as a Cetra after reading the reports in the Shinra Mansion. His will, perhaps blended with Jenova's, was as strong as ever, if not stronger.
Even after going through his time in the lifestream Cloud could still kick his ass. Sephiroth is by no means ridiculously strong, Jenova treatments of SOLDIERS just wasn't very well developed in his day, and he was strongest.
His will wasn't strong, he let Cloud beat him in the reactor. Cloud at the time was at a severe disadvantage.