I don't believe the springs are smaller since the retainers are a perfect fit with the Z1/KZ springs, I used 650 parts to do a poor boy shim under bucket on the motor I put together last Spring. The springs may be a lighter rate since the shim under bucket is lighter and would require less pressure to close them? As far as setting valve lash with S.U.B., it's not bad if you take good measurements and notes of shim locations, and are familiar with setting the cam timing. However as H.R.M. has said, it does open up the chances for a " mishap" more than the traditional 900/1000 setup which requires no disassembly. We got a lot of practice setting valve clearance in Lincoln when the 650s first came out chasing valve noise. I remember being less than impressed that the new 650 didn't have cam bearings like it's big brother. (like I knew anything at the time!)
Mike
Former M.E. at Kawasaki Motors Manufacturing, Lincoln, NE
1966 W1 (the Z1 of 1966-50H.P. and 100mph!)
1974 Z1
1978 KZ1000 LTD
1976 KZ900B pile O parts
1980 KZ750E
1980 Honda XL250S (I know, wrong flavor!)