Just a word of caution regarding exchanging carb inserts in 2 needle carburetor...
If you go down from a let's say 8mm insert to a 7mm insert, the low-end needle and spraybar remains on the same position and because the reduced diameter of the insert, is located nearer to the projected surface of the carb insert.
Having it them nearer could lead to carb adjustment and fuel atomization problems, just remember that fuel converts into a mist entering on the engine.
If you have a 3 needle engine (the ones with the 'brass screw' you can offset the spraybar and modify when you want your carb being driven from the LSN to the HSN. C6 Engines comes with a 2 needle, don't know why they switched from 3 to 2 needles on their high-end engines where you want the maximum flexibility and performance.
Be forewarned that smaller inserts mean slightly more runtime and better low-end, but some engines react very bad to them and start being a bitch to tune them.