Jet kits? Buy them or not

I was searching on the web on whether or not to use a jet kit on the VFR's. Many people say not to use them because they are hard to tune.

So from my research, people say to keep the stock needle and replace the 2mm washer with a 3mm. Also turn the mixture screw 2-2.5 turns out, and should be a good setting for those who have a slip on with KNN. Stock 128 mains are just fine.

Does this sound right? Or should I buy a jet kit when I buy my slip on?

Also I've noticed that there is a flat spot between 6-6.5k rpms. What causes that? How can I fix it? That is one of the other reasons why I wanted to buy the jet kit.

Thanks'