A longer barrel aids accuracy when using iron sights, the longer sight radius is better. If you are using a scope then barrel length is basically a non-starter. As for velocity check out this sight-- http://www.ballisticsbytheinch.com/22.html
. It shows that velocity generally begins to top out at about 18-20" with gains on up to 22" (look at the chart using actual guns.) I did a similar upgrade to my 10-22 a few years ago and the thing that gave the single biggest improvement was upgrading the trigger. Have fun!