I'm 5'7" and I had the same problem; I went with the Wild1 500 bars

 

I got the 6" rise Street Slammers with internal wiring, really layed back and comfortable ride positioning, I am 5 11".

 

I am 6'1", 205lbs and have an 07 standard, when i first got it back in 06 i got rid of the stock set up and went with the NT setup. Aside from replacing the cables for the shorter setup, they fit very nice, after a couple of rides i too had some muscle sorness, so i put on some 5.5" pullbacks and that worked better, but still a little bothersome. So i got some of the 6.5" and thats what i've been with ever since. Just changing the risers from the stock 6" to the 5.5" or the 6.5" pullbacks i had no problem with the cable length.

 

+2 on the apes. Best mod I did.

 

I'm the same Height and used to have an FXST with drag bars, same setup as yours.. here is my good feedback (as i had lower back pain just like you).
1 - The strange thing is the Lepera Barebones did not give me this problem is it because my *** hurt more ? not sure, but my guess is it put me lower and closer to the bars.
2 - I also have a setup with spring seats that put me 2" closer to the bars and higher, that solved all issues.
3 - I also changed to 12" apes with 1" risers but mine are 'Z bars' as you can see and they go back and down a lot, and that too took away all my back problems, but that required changing cables.

The worst pain came from the Mustang Seat it was comfy but put me really back and not high nor low .. seemed to affect my back the worst.

Oh, I used to have the 6" up and 3" back risers on my old setup, you can see all of them on my albums.

I hope this helps.