So right now I'm running 15/42 with 17" wheels and I lost some top end from switching over from the enduro setup. I was running the exact same gearing with 18/21" wheels. Anyone know where I can get a smaller rear sprocket that'll fit the 690 cush hub?

 hmmmm. I dunno. I used to cruise 80 mph at aobut 5k and now it takes 6k to get that outta her. Suppose I didn't lose too much. Just feels like she has to work harder to do the job she's used to doing. Figures about right. Runnin' the same pace, just with shorter legs. 

Interesting.  I'm puzzled.  At the risk of asking a dumb question, did you reset your speedo for the new tire / rim combination?  A 15/42 setup should feel like you gave up first gear and added a 'seventh' gear based on the shift points.  I'm running 16/45 which is the same gear ratio as you're running.  My minimum speed in sixth gear before she starts to bog down is around 55mph.  Yours should be about the same although I'm beginning to suspect the shift points may not be identical although the ratios are the same.  I'll be doing some experiments to see if my hunch is correct in the not-so-distant future.  (I'm going to try the 15/42 combination and see if there's a difference in the shift points from the 16/45.)  My hunch is that there is, but I have no evidence to back it up yet.  Just a theory)