If I remember correctly a one tooth change on the front is about 500 rpm difference for the same speed in top gear.
I think you'd have to specify the speed to calculate the difference in rpm a sprocket tooth change would make.
Let's say at speed, "X" mph, the engine turns 4,000 rpm with a 15-tooth rear sprocket.
Change to a 14-tooth, and it will turn at 4,287 rpm at the same speed.
Change to a 13-tooth, and the rpm will be 4,616 rpm; again, at X mph.
At 5,000 rpm it runs, stock, at Y mph? With a 14-tooth front sprocket, expect 5,367 rpm; with a 13-tooth, 5,770 rpm.
Formula: New rpm = Old rpm X 15/(New Countershaft Teeth Number)
If these figures are in error, either I've made a mathematical mistake, or I don't understand drive ratios, or both!
A smaller front sprocket shouldn't cause any mechanical interference (although a larger one (say, 17 or more teeth) might pose some clearance issues), I'd think; some minor additional chain wear may result from the tighter radius of a smaller sprocket; and if you loose too many teeth, conceivably, you could run out of chain slack adjustment room with a too-long chain (cut some links to solve that problem).
Welcome, indeed, wv stump jumper! I've jumped a few stumps myself, at your Hatfield-McCoy Trails; and plan to jump some more at the Shenandoah 500 next month!