I just replaced my Nighster fork springs with Progressive suspension springs. The part number is 11-1527 and although I was not going for additional lowering, I can see that it is possible by reducing the amount of spacer that is placed on top of the spring. You can also lower your fork spring by shortening the stock H-D spacer. This would save some $ over buying new springs.
I also changed the shocks to progressive 412's and the ride quaility is about 600% better than stock. As the temp here was only about 0, my ride was somewhat short, but in the 2 miles or so that I rode, I can tell you that the shocks now have effective damping, and the progressive fork springs got rid of all the sag in the front end yeilding a ride quality that is great! As a side note, the progressive 412 is available in black and really looks good as well.
If you choose to go this route, and will be doing the installation yourself, watch out for the sharp edges on the stock H-D spacer at the top of the forks. It's sharp enough to remove a finger if you are not careful. I used welders gloves when removing the top cover off the forks and had no issues.
It took awhile to find the 1527's online but I just ordered them, then was snooping around to see if anyone else had done them on the nightster. It's not that big of a deal on the 1200N is it? Did the springs come with an instruction sheet from Progressive? They didn't have anything (that I could find) on their website for our bikes. I've got an '08 that's already got progressive 440's on the back. They sure improved the ride and then when I put the ME880 tires on, it was a real difference from the stock shock and awe ride. Did you have to get a special tool for the top cover?