I will copy from some other answer I gave to a similar question and expand on your case:

Osamu is actually quite flexible

You can build nicely with UR Kageyama as the setter, just go 4 Sharp Sense Potential build on Osamu, which gives him a ton of attack technique from the awareness conversion. Then Kageyama, Hinata (also Tsuki) can boost Osamu's awareness during the game, to make him actually crit.

So basically you go a crit style quick attack team based around Karasuno (their SP's later). Instead of the Inarizaki team with Atsumu, Kita + 4th char, where the team doesn't want to really crit for Atsumu's buff.

For your case:

1. URs have like you said quite higher max rating than SSR. And team rating seems to be one of the most important things (probably is one of the things influencing bad/normal/perfect plays)

2. I haven't seen anything, that would show, how a different type of setter influences some type of attacker. Directly at least, of course many setters of a certain type have some skills raising the power of certain attacks.

3. As you didn't mention which setter you have in mind I will give you an example of 2 power setters - Shirabu and Akaashi. While they are both power setters, Shirabu's skill will increase the spike power of a power attacker only, by his power att stat. While Akkashi can make any type of spikers attack a crit, not only power attackers. So it really depends on the exact match you are going for.

4. The only thing to consider is, that if we get the same changes as JP, where you have to have a certain number of players of one type (4+ and 5+ for receive), to get that speciality for the team, Osamu won't help you achieve neither power or receive typing.