Jim:

Most of the Saito engines will run up to 15K or so before floating the valves.

The late FA-120S has springs that are much stronger than the earlier ones, as a guess you could run an old FA-120 over 16K using them.

But why bother? Torque peak on most is under 10K, hp peaks around 11K, so turning them higher gains nothing other than increased wear.

Bill.