Well, the thing is, we have examples of several super strong Jenova injectees, but we also have several examples of Jenova infectees and injectees who do not possess this superstrength. So if there is a superstrength element to be had from Jenova, it must only happen in rare circumstances.

As for Kadaj becoming the more powerful Sephiroth, remember, Kadaj was Sephiroth, and Jenova cells were his body. He needed that amount of Jenova cells at once in order to be able to assert his will enough to project his complete will, as opposed to the fragments.