So, let me start by saying that I stole this idea from a comment someone else made here recently,but wanted to post up something I tried that worked well for me.
At 15,000 miles, I had the same crappy low-RPM, low speed driveability that almost everyone complains about. The bike wasn't happy trolling along at 50-55 or so in 5th or 6th gear (even with a 1 tooth smaller front sprocket) unless the RPMs were in the sweet spot 4000 or above. I always had it in the back of my mind that even my temperamental, comparatively high strung RC51 putted along more smoothly than my Vee. I figured that there may be more to it than the lean spot, TB sync fixes that we have all tried and definitely help.
With the bike on the center stand, I could grab the rear chain/sprocket, and move it about 1/2" forward & back. Riding the bike, you can see how the power pulses from a v-twin at low revs and that slop could cause the sprocket to hammer back and forth against the rubber cushions.
What I did was to very simply pull the rear wheel, remove the sprocket/sprocket carrier, and cut rubber shims out of an old inner tube to place between the outside of each cushion and its pocket in the hub. 10 shims in all. Essentially, you are just squeezing the "c" shaped cushions closed a bit so that the sprocket carrier fits more snugly inside them.
The improvement is amazing. I'm not talking about chugging along like a cruiser at 1500 RPMs, but now on a smooth, flat stretch of road, I can click into 6th gear without the bike protesting.
Well worth the 30 minutes and $0 investment.