stiffening the rear preload will have the same effect as putting a larger rear sway bar on a car. The bike will turn quicker and the steering will be a bit lighter. But upping the spring preload will also make the rear ride harder and stiffer.
My advice is to adjust it one notch up and see if you like it. If you do, keep bumping it up another notch until you find it too stiff/hard for your liking, then back it down one notch.
if you are finding it "mushy", upping it will make it feel crisper.
The steering geometry on the 250R is quite conservative and adjusting the preload should not make you tuck... but check your tire pressures and make sure they are with in the recommended ranges.
interesting cars.. you should post in this thread.
http://www.ninjette.org/forums/showthread.php?t=9815