The rating reduction is for hitches NOT using WD.
We had a extensive thread on this a while back, and Barney the moderator got some manufacturers involved, and Ron Gratz has also weighed in on the issue.
I went from the 12" to the 18" drawbar probably 4-5 years ago. The only thing I noticed was a slightly bumpier ride while towing relatively lightly loaded.. This makes logical sense as the longer draw bar has the effect of increasing the leverage of the WD bars...(makes them act stronger) So you may need to readjust your setup when going to a longer bar.