Ther standard springs are dual rate (sharp blended) which puts them somewhere between straight linear and progressive. The average user might not notice a change from that to progressive. A track user might nott want progressives, prefering the predictable linear action. A well groomed skilled track rider on the road might well prefer progressives because they iron out a lot of the surface variations (not experienced on tracks) and have a gradually increasing response to more intense compressions, such as riding over larger undulations at speed (with the damping holding off the rebound.
I push quite hard on the road (nothing like a track rider though) and find the standard mixed/dual rate springs to be adequate. I was considering progressives for my mk2, but the spring length of the mk2 is huge compared to the 9er's.
Before changing springs on the 9 at least try turning the spring preload to maximum and using fresh slightly heavier weight oil, the front preload adjuster has a more noticeable effect on the 9er than the mk2 due to the more compact, stronger spring in the 9er. The 9er springs allow a lot of sag (for comfort) but can be quite good set for minimum sag
Regardless of the springs you would feel very little improvement if the rest of the suspension is badly set up.
The best thing to do first, is to turn back all damping front and back and then set static sag with the preload adjusters (I'm going to set my 9er up at the weekend for 25mm sag front and 30mm sag rear). Then introduce rebound damping enough to allow just one bounce at the front and back ends when you heave up and down on the frame,(If the frame wants to bounce down again after heaving it upwards then there simply isn't enough rebound damping and the shock needs work)...on the rebound the rise should feel controlled as you lift up on the frame.
Then introduce compression damping to suit the surface you're riding on. This would be a good setup for a smooth track with some strong undulations (or fast good roads)....for less spirited riding you can drop the rebound and compression back for a more comfortable ride, but once sag has been set you shouldn't need to change it, unless it is to adjust ride height. (The spring preload adjusters don't adjust the amount of springing, only the ride height)
Once you've got the basic ride height and compression/rebound damping sorted the difference will be huge (over a poor setup), If after that you still need that bit of extra comfort at the front then you could try the progressive springs.
I'm only going to town with this response because last weekend I had 2 interesting conversations - 1 local biker I gave a lift to , to buy a secondhand Fazer. He loved it, the rear shock was nakered and he said "It's so lovely and smooth.....when I pushed down hard at the back and lifted my hand off the seat quickly the seat was always in contact with my hand...no rebound damping, so that means that when the back end hits a large bump in the road the suspension will compress hard and fast and then without rebound damping the energy in the spring will be quickly unleashed and throw the seat up, making the bike unstable....that upward momentum could also make the rear wheel lift off the ground.
The other chat was with a guy who was so light he couldn't get enough sag on his bike because the previous heavy owner had fitted a stronger rear spring....He mistakenly thoguht that the preload adjuster made the spring softer, so he wound off all the preload at the rear and had to turn compression damping right down to allow the back wheel to come up enough over bumps, but again he was having a bumpy ride.
I used to have the same issues until someone took the time to explain the way forward to me, in this order
1) set sag front and back with preload adjusters, on a bike with 50/50 weight balance set the front sag to be about 75-85% of the rear sag (If you can't achieve this sag you have the wrong springs for your body weight)
2) set rebound damping, enough to tame the spring rebound so that the rear and the front rise steadily at about the same rate (If the rising rate after bouncing the seat down is too fast then the shock/forks are dead)
3) set compression damping for best feel over the road surface - usually a similar number of 'clicks' on the compression adjuster from maximum, as the rebound adjuster is a good start, and then slowly reduce that for the most comfortable/responsive setup for you. (if the bike still feels bouncy the shock/forks are dead)
But then thats when the real science begins, because you need to look at the rear tyre wear patterns to fine tune the suspension. but by this point you will have suspension already at 90% at it's best setup, and because roads vary so much there might not be much point 
There are many other ways to set up suspension, this is just one that has worked for me
Edited by fixitsan, 28 April 2017 - 10:08 am.
