I put the "up" bars on my 2005. I had to have +2" cables, but in your case, you will may only need the clutch cable. Maybe a brake line. Ergo says you do not need any cables at all, but I can't see it - at least not on the older bikes. Maybe the Rushmores are different. The "back" bars position the controls the farthest from the original factory position, so they would be the most likely of the three Ergo styles to require extension. Like the dealer said, you have to have the bars to know.
I liked the Ergo bars a lot. One thing you should consider that they are VERY straight. Your wrist position is almost like a drag bar. I used to joke that it made it feel like I was riding an 800 lb dirt bike. If you are really broad shouldered, you'll probably like them, if not they may stress the outside of your wrists.