OK, I fitted the 18" Samson heat shield and it fits reasonably well, It covers the entire straight section of the pipe.
Of course the real problem area is just forward of the upward bend under the swingarm bolt area, as that is both the hottest part of the pipe (because of the bend restricting flow) and also where your leg is most likely to touch it when sitting astride the bike
However extra cannot hurt and it actually looks good.
In my case I just bolted the shield on over the top of the old screw mounts. You would get a neater fit by grinding off/removing the old mounts however unless you are building a show bike it really isn't worth the effort. Mounting was a 5 minute job.
Overall for a 5 minute job it makes the bike safer and more practical to ride and also looks better than the stock shield. Well worth doing.
http://www.samsonusa.com/H-113.html