i've heard there are issues with wire resistance and the throttle acting wierd. not so sure about another 6" of wire causing problems but i'm not an engineer.
This has been talked about. Do a search. I can't remember when. It's been in the last 2 months. There were a few who figured the 08 drive by wire thing out. They said there was plenty of extra wire inside to use.
My 14 inch Flanders buckhorns are on my FLHR in my pic below... One nearby dealer would do it, another wouldn't. I've ridden about 1000 miles with it so far, and no troubles. Throttle is a bit lagging, but it's the same whether the cable is long or short. I've heard there will be a fix coming soon.