The 600 loves her gas, especially at higher speeds. As it's an XC, I'll assume that it doesn't have the m10 suspension, so that's not causing the problem. I'm not fully aware of the setup of the 600 on an XC, or a 2002 at that, but I do know perhaps one of the best ways to improve mileage is to modify the clutch (clutch spring kit) to help prevent over-revving and improve gas mileage at those higher speeds where the 600 typically chugs her fuel.
Limiting your speed to 45mph should render better mileage... but this is an XC we're talking about... if it's anything like the 500 you'll see your best mileage ABOVE 50mph, not bellow it. My brother's 500 XC/SP 2005 model doesn't like any speeds bellow 50mph, but keep it above 50mph and unless we're on hills he gets better mileage than my CFI, vs typically my CFI gets better mileage than his 500 carbed.
I'm not sure how modifying the carbs improves mileage... I'm aware that it can be done, but there's the risk of damage along with it. Obviously the stock setting will give you longetivity. But, 10mpg is normal for the 600.
I'd do a little more research... I don't see how idling at 4000 rpm can give you better mileage... it's got to be over-revving. Besides... all the sleds I know clutches engage before 4000 rpm anyway... how was it not running away on you??? Check your belt for wear, it can affect your mileage as well.
Hopefully... something I've said will help. For the most part, ask your local dealer for options to improve gas mileage, and see what they have to say. It's worth asking.
