You can take all the guess work out of this with a simple test. Take a Sharpie and mark the primary clutch with a straight line from the center shaft out to the outer edge. After riding you will see how far up the clutch your belt has been riding (shifting) by how much of the line is worn off and this will tell you where you need to be with your gearing. If you are not shifting all the way out (I would only to go to around 3/8" from the top on a high speed run) you are geared to high and should gear down to get the maximum out of your sled. In simple terms, the clutching is your transmission and similar to a truck, if your diffs are geared to high you will not be able to pull 5th gear. Gear down and you will get this back. The reality is if you are geared too high, you can gear down and gain much better throttle response, acceleration and NOT lose any top speed.
It is much simpler than most people think, you just need to take the time to verify it.
The reason the OEM's gear the sleds high is that if someone takes a stock sled on the lake with a very smooth hard pack and winds the sled out to maximum speed you can shift out to the top of the clutch and to go any faster if conditions would allow for it you would experience an over-rev condition. They protect from that by gearing high so it doesn't happen. Practically, most people would/could never do this, particularily in higher snow or mountain riding so there is a lot to be gained by gearing down.
Cheers,