Slip ons save a serious amount of weight but don't add any performance. Without a tuner you will actually lose a little power compared to the stock exhaust.
A full exhaust, K&N air filter, snorkle delete and tune
Or slip on, cat delete K&N air filter, snorkle delete and tune
Will yield the most power
But you can snag some power and better throttle response with just a tune and K&N air filter.
The Slip ons will also make the bike sound better.
I personally after reading about what works and doesn't work on our bikes, past experience on other bikes and after talking with Kerry at Area P am starting with a slip on, Area P tuner, K&N air filter, snorkle delete.
With just my slip on I can feel a loss until about 7k Rpms after riding it twice today.
As far as the levers go, they allow you to set the distance from the grip to the lever.
This is advantageous if you have big or small hands to get the engagement for braking or clutching exactly where it is most comfortable for you