This is a simplistic view, also, as it ignores a lot of the other benefits that Kane brings to the team that our strikers do not. Positioning, heading, dribbling (underrated), hold-up play, passing are all far superior to anything the other players provide, which will result in the team being better as a whole, leading to more creativity and possession.
I think the improvement will be a bit bigger than 9 points (performances for one would be better, making it more sustainable), but let us assume that we got 9 points from signing Kane as it isn't ridiculous. That is actually a massive improvement when you put things into perspective. If you added that 9 points today, it is very realistic that we would end this season in excess of 80 points, maxing out at a possible 84 points in the next two games. In other words, we would be level with Arsenal on points if we won our last two; Arsenal have a cap of 84 points also. (We have a game in hand).
Assuming City win their last game, we could have finished this season only 8 points away. This would be seen as an incredibly promising league season, and we wouldn't be a huge way from City, to be honest. There is a decent chance that we may have even topped the table for periods during this season. It may not sound exciting, but that is a huge difference to what we are currently.
There would need to be work as, no, signing Kane won't be enough, but 9 points would be massive. We could then look at a midfielder and whatever else is needed.