I had an 08 Busa for 3 1/2 years. I had some old injuries and then 3 buldging discs in my lower back.
I put 1.5" spacers under my handlebars. An improvement, but not enough. I then added Heli-bars, and had to cut the spacers to .75".
The 1.5" spacers, and the .75" spacer/heli-bar combo both cleared the inside of the stock gen2 fairings and winshield and used stock length brake/clutch lines.
Both were an improvement, but still not enough for me. I also tried 5 different seats.
Then, I bought a Spiegler bar conversion. A big improvement...but still not enough for me.
The Busa seems like it was designed for someone whose arms were twice as long as their legs. The gas tank is simply too long for me to be comfortable. The Busa does everything well, but I find it's riding position too agressive.
I traded the Busa even for an 08 B-King, and didn't regret it at all.
The B-king was heavier than the Busa with a higher center of gravity. So I sold it after a year and a half.
I will be buying another GSXR 1000 next. Even with a bad back and knee problems, I find them much more comfortable with a shorter reach to the bars.
My friend has an 09 GSXR 1000 with dirt bike(Spiegler) bars, and it the most comfortable bike I've ever ridden.
I'm also 5'10", 170lbs, with a 33/34" inseam, and I can flat foot the Busa and 1k easily. The B-King had my heels off the ground.
The B-King can also have the ecu flashed to a Busa, the exhaust mid-pipe opened, and ram-air added. This will make it just as powerful as the Busa, as it's the same engine. It's a great touring/cruising bike, but as expected reaches it's cornering/knee dragging/ handling limits much sooner than a Busa and then a 1k.
If the Busa is bothering your back, I'de get rid off it before spending alot of money trying to get it to where you want. As you may be able to, or you may not.
Good luck with it.