My idea was to try the 15 tooth cog already on your bike to determine if it will give you the speed you want. I'd find a straight, quiet, flat road, like the access road behind a big shopping center, and crank it up to full speed. Then find a steeper-than-average hill in a quiet neighborhood. You might find that the stock SS setup on your 925 is the best compromise. I took a 50 mile ride on this bike last week, with a fair number of hills to climb. It was a good setup for that.