Bumping this back up to the top - suspension is important
I spent a bit of time riding a few loops of varying terrain around PHX on my new-to-me '03 640 Adv. My '01 adventure was softly sprung and underdamped, but it felt a bit better than the '03 - and I have no idea why. It's clear that the suspension is nowhere near where it should be for handling the Adv's weight, fuel load and a heavy rider (I'm 240lbs geared up).
If I rode prettly slowly, I had a plush ride, but I don't have the self restraint to ride that pace for any length of time.
I rode some technical stuff, rock, sand, etc, and the general consensus was I had to do something about this asap. It was chronically bottoming the fork - it's a bit unsettling to hear the front knob rubbing the fender while in 6" of sand at speed. The fork would actually bottom while braking hard on pavement. The rear was also pretty odd feeling, but it was hard to pin that down since the fork was so poor. In general, I really couldn't wick it up because the suspension action was so poorly damped and sprung for the weight it had to deal with.
I called MXTech and talked to them about it. They've done quite a few LC4's, and since it was fairly closeby, we decided to take the bike there and size it up. So down to MXTech I went. After making a lot of measurements and chatting about riding style and such, it was recommended to me to stick with the (original?) rear spring since the static sag numbers were actually quite good, but the shock valving needed to be changed rather dramatically. I didn't check the # on the spring since we decided it was OK. I am assuming it is the 7.0 OE spring.
Up front, we decided to go with .50 springs and revalve the fork. I declined on the preload adjusting caps on the fork, because quite frankly, for the purposes of what my 640 needs to do, I'll never do it reliably.
Anyway, even if you're a prettly light fella, and you feel the fork spring rate is OK, the fork needs some valving help to really get the max benefit of the WP. The shock, at least on my bike, appears to be fairly well sprung, but poorly damped.
My point is, if you're actually sufficiently dissatisfied with the suspension's action to consider re-springing it, you really ought to consider revalving the shock and fork to really take advantage of what the WP suspension really has to offer. Revalved, resprung and the sag correctly set makes a world of difference, but it ain't cheap. It is, as Flanny says, the best thing you can do with the bike.
Thanks to Flanny's post for making me decide to do it rather than dealing with it as it was.