Changing gearing had no effect on my speedo's accuracy vs. true. Only when I changed my rear tire did the indicated speed actually become closer to true.
As for gearing change, I'd not recommend anyone who is heavier, or rides more aggressively, to consider going much more than 16T front sprocket and the taller (/55) rear tire.
If you are lighter, on flat land, low altitude, no passengers, or not very heavily loaded then you could consider the smaller rear sprocket as well. I meet none of the above criteria so I am staying where I am now (16T & /55 tire). I can cruise at 80 mph and be in the vibe free zone. Roll on is still good (also helped by Ivan's reflash) and peak acceleration is better than stock (thanks again Ivan).