But wait, there's more...
If there were just springs in those fork tubes that would be the end of the discussion. But there's oil in them there tubes. Fork oil is used to damp out the motion of the springs; without it if you hit a bump the springs would compress and rebound repeatedly, and your bike would go wallowing down the road until friction damped out the springs' motion or other inputs from other bumps caused the springs to start boinging along in a different way. This damping is created by forcing the fork oil to flow through small orifices inside the forks. There are different ways of accomplishing this- expensive ones involving shim stacks and adjustable springs and valving, or inexpensive ones like the damper rod fork on the EX500 and most other bikes that come with conventional forks.
The damper rod is basically a long tube with a few holes drilled through it crosswise. As the forks compress, fork oil is forced through these holes in the sides of the damper rod. The end effect is to create resistance to flow, which controls the rate at which the spring can be compressed. Upon rebound, the oil flows back through a different path with smaller passages, creating much more resistance to travel. This rebound damping is what keeps the spring from oscillating and gives you a degree of control. Without adequate damping the wheels tend to thrash around in an uncontrolled fashion, keeping the tires from maintaining good contact with the pavement and giving a rough ride and a bike that tends to drift to the outside of bumpy turns. You can find a good, in-depth description of how a damper rod fork works here: http://www.racetech.com/HTML_FILES/DampingRodForks.HTML
So if you stiffen your springs by shortening them, you'll also need to increase the damping by a similar factor. This is why shortening the springs is only half the job- you need to also change your fork oil. Unfortunately, I don't have any nice easy formula for calculating this. Perhaps someone from the racing community, who has some experience in this area, can expound on this.
One final note on damping. As the velocity of a fluid traveling through a restriction increases, the resistance to flow increases to the fourth power. So if you double the speed of the oil traveling through a damping orifice, the resistance to flow increases sixteen fold
. This is the real shortcoming of the damper rod fork. Get your oil thick enough to assure good control and ride quality over reasonably smooth pavement, and you run the risk of hydraulic lock when you hit that square-edged pothole. Thin out your oil so that the forks are compliant over rough stuff and you end up with a wallowy rocking horse of a bike.
What to do? You could install a sort of one-way valve that was spring loaded. Under normal conditions it would stay shut and you wouldn't know it was there, but hit that pothole and the sudden increase in force pops the valve open and bypasses the damper rod, allowing freer compression of the spring and better ride quality and control. Upon rebound it closes, returning the rebound damping duties to the stock internals. That's what a Race Tech Cartridge Emulator does. It has nothing to do with keeping the springs from bottoming. And no, I'm in no way connected with Race Tech, I"ve just had really good results using their Emulators in a couple of SV650s I've owned.