This looks like a nice coupling for 8-to-10:
http://www.fscoupling.com/8mm-to-10m...-25mm-p-214.ht
Its rated for 19,000 RPM, but my physics knowledge is dusty at best:
2 Newton Meters of torque is about 1.5 foot/lb... which I have no idea how to apply here. (AKA Idon't know the math to figure out if its going to rip in two!)