I just went through all this last week. Heres the research. I did waste money on a jet kit. didnt need to.
Read these. twice.
specifically this chart is useful-
First off, they speak truth about the brass screws- they strip out immediately. use an ez-out to remove, then get the nice allen head stainless ones from this guy-
Next, here's my finds. I have V&H cruisers, the GAK(pod filters instead of stock airbox), and the needle clip in the 4th groove, with 125 jets.
NOTE- mikuni jets and dunojet jets use a different numbering system!!! I couldnt find any cross reference between the two.
Now, after hours and moneys spend fiddling, I can say that I think there is no performance increase. Testing with a smartphone 0-60 and 1/4 mile app shows *maybe* a .5 second 1/4 mile time difference. The bike is a bit louder- you can tell that its sucking more air in- but doesnt seem any faster. time will tell on fuel mileage difference.
now, to be fair, testing was not perfect conditions. the app is glitchy. Also, control was my buddies 650 with stock intake, jets, and V&H slip-ons.