You can figure approximately 2-4 fps per inch of barrel.....until you reach the point of diminishing return. To my knowledge TC never made a longer barreld rifle than what they currently manufacture.
Are longer barrels more accurate.......well....no. At least not the lengths we're discussing here. We generally shoot longer barrels more accurately because of a little thing called sight radius. That's the distance between the front and rear sight. Generally a longer sight radius is more accurate for a shooter than a short one. In revolver shooting this is very, very apparant to me when I switch from a 3-4 in. barrel to one of my 7 1/2 in. barrels. Also, for some of us, me included, a longer barrel is easier to shoot accurately offhand because we believe it "hangs" better than a short barrel. And you know what they say about opinions......