Hopefully this summary will help to group a bunch of scattered replies on the topic. Obviously, it's all JMO.
I found just a re-springing of the stock shock to be a very adequate economy answer to the wallowing and general lousy handling experienced on these as you start to push them, ESPECIALLY for just street use. It's NOT a Penske but is still a dramatic improvement. If you're not experiencing those wallowing problems, not pushing it a bit, or just a cruising type rider, ignore this post.
I took the time honored approach that on a budget, the best bang for the buck is get the spring right. These are so soft that that approach made even more sense the more I thought about it. I took the measurements, figured the linkage ratios, drew up some cups, sourced a used spring, and proceeded. I was AMAZED with the improvement and easily recommend it to anyone who wants to improve the suspension but hasn't the money to do it "right" (translation: Penske, translation: $).
I've used this on the track and will again, finding it totally rideable, but the shock absorber's weakness really shows there, cold tearing tires. Inflating a couple of extra pounds over typical track pressures will help, but not totally cure that. If you're a rider who track rides and can push that hard you could probably justify the cost of a Penske just with a season's tire wear savings alone and could realize the benefits of a nice, tuneable shock.
But if you're a street rider, possibly a novice track rider just getting into it, you won't have the tire tearing problem and will LOVE the dramatic improvement in handling if you're riding twisties even a little hard. On the street, even experienced riders won't be able to push so hard as to have the cold tear issues and will find that it's rideable at a pretty good pace, certainly FAR better than a stock set-up. Not a Penske, but not so bad.
It requires a spring for your weight, a set of special machined spring cups and, not necessary but highly recommended, a set of links (often called dog bones) to raise it.
The spring is a 2" X 6" series from Hypercoil and is the same spring used on Penske's shock. They are available in 50# increments. The range most likely to be used on these is 400, 450, or 500#, and for the really husky ones, maybe a 550#. New cost, about $100.
The spring cups are machined from 6061 aluminum and should cost no more than $100. I can make them for that, shipped. That price may be better depending on material costs.
The links to raise the rear and built with a minor outward step in them to clear the spring's slightly larger diameter should cost no more than $65. If you weren't raising the rear the stock links can be used but a washer should be added to each side to afford just a little more clearance. The addition of the washers then requires a longer set of bolts to maintain the engagement of the locking feature on the stock nuts. The spring will just
fit between the links either with this mod or a purchased Penske, but even the slightest shift of it on the spring cups or the smallest variation in diameter will cause it to rub. Not good. The additional 1/16" per side assures clearance.
Summary: The cost for the shock mods alone is about $200.
Add the links for a truly complete package and the total is $265.
Shopping for a used spring from a racer who has had occasion to change his Penske shock's spring could be as little as $25 shipped (that's what mine cost) so you could conceivably complete the whole deal for under $200. If you have access to a machine shop and can figure out the parts, maybe way
less than $200.
Next lowest cost package to my awares is a basic Works Performance shock, ordered and sprung for your weight, at about $400. That's an old price and from memory so it may be more, not likely less. It would still need the links if raising the rear was to be done so add the $65 for those to complete the package. That makes a total of about $465.
The ultimate, the Penske, sprung for your weight, is around $875. It has everything including adjustable ride height. (For those not familiar, ride height adjustment is NOT the same as spring adjustment. All of these, including your original, have spring adjustment, but not ride height.) At its lowest setting it already raises the rear 7/8" but can then be adjusted up from there. It should
have washers added to the stock links to assure spring clearance and then longer bolts to assure the locking nut engagement. Those longer bolts will not be hardware store stock in that size and length so will have to be ordered from a fastener supplier. Expect between $10 at best and $20 at worst by the time you cover shipping and handling, getting that package up to just under $900.
So, there you go. A good list of the options... for the rear. ;D If you're going to do the rear you should really consider doing the front, too. The best handling is when you keep the front and rear working similar. With a spring change at the rear that will be at least a one third increase for anybody but the lightest rider (the stock rear is 300#) and it will underscore the front's soft springing.
At the front expect to spend $150 for parts to do the springs alone. That would include the springs, fork oil, and a little for miscellaneous. Add another $150 for cartridge emulators and you'll have all of the parts for the front, as good as it gets. No high dollar Penske options here, thank goodness.
The works for $300, the minimum for $150, or maybe shop the racers again for used.
This list is from my experience with my bike over about 10,000 miles riding as modded, riding it on both street and track, about equal amounts of each. (I use it coaching novice sometimes.) The total list of mods is:
1. re-sprung rear.
2. links to raise rear (started at 7/8", now at 1 1/2").
3. re-sprung front.
4. cartridge emulators front.
5. front raised 1/2" by sliding tubes in triple tree clamps.
6. radial tires.
7. lowered stock bars (approx. 2") using old 600 risers.
8. carburetor pilot screws out 2 1/2 turns.
9. EBC front brake pads.
10. EBC floating front rotor. (Just installed, warped two stockers beyond hope.)
That's the TOTAL list. And in that configuration it can be ridden quite fast without doing scary things.
If I were racing it or running against my lap timer at track days, going for the track record,
I'd certainly want a Penske. If one comes my way at a steal, I'll buy it and put it in. Other than that, I'm content with the bike as it is.