As a bigger guy on the severely undersprung EX500, I have personally experienced the huge difference that springs of the proper rate can make. When the springs are too soft, the wheels flail about uncontrolled under the bike. The force from bumps isn't properly absorbed, so you have a rough ride. When you're riding hard, the springs can't support your weight against those forces, making it feel mushy in corners and such. With springs of the proper rate, the wheels move in a controlled manner like they're supposed to. This means bumps are properly absorbed and the bike doesn't squish down when riding hard. It's more comfortable and performs better too. Due to the extremely soft fork springs used in the PreGen, I think everyone can benefit from an upgrade.

Here's a chart of ideal weights for different spring rates from RaceTech's calculator (http://racetech.com/ProductSearch/2/Kawasaki/EX250%20Ninja%20250R/1986-2007). This is just a quick reference so you can get an idea of what spring rate you should be looking for. Don't take RT's numbers as gospel, and different people may prefer slightly softer or stiffer springs, but this will give you a rough idea of the rate to match your weight. Mostly it shows how bad the stock springs are.

RATE (kg/mm) STREET (lb) RACE (lb)
0.44 (PreGen) <30 <30
0.50 51 35
0.55 84 67
0.585 (EX500) 108 89
0.60 118 99
0.65 (NewGen) 151 130
0.70 184 162
0.75 218 194
0.80 251 225
0.85 284 258
0.90 318 289

For example, if you weigh 250lb, EX500 springs aren't going to help a whole lot. If you weigh 100lb, NewGen springs will probably be too stiff for you.

I myself have done the EX500 springs, and cut them and set the sag, I like the fact of the NewGen springs having more, this means not having to cut so much off.

In my case using a 1st Gen EX500 springs which are 20.5" I cut 5.5" off to achieve the desired results.

PreGen Ninjette springs are 18"

new spring rate = (original spring length / modified spring length) * original spring rate

20.5/15*.585 = .7995 kg/mm

Here's another handy chart that I saved and use all the time. The caveat is that for street duty choose a rate that is a full step lower (eg. 0.9 -> 0.85) as the curve is biased toward racing.

https://dl.dropboxusercontent.com/u/180639/spring%20rate.gif