....... If preloaded, will they bottom out sooner? .....Has anyone changed the oil in their rear shocks to improve their worth, or do we replace them?
Addiing preload will do the opposite, make the forks harder to "bottom out"
I installed the Progressive springs, and they require you to cut down the stock spacer as they are longer springs. Progressive reccomended cutting the spacers to 7". A few others , along with myself.... left them a bit longer @ 7-1/4 to 7-1/2 "...providing more preload. You can retain the stock springs and simply add say, an inch of the proper sized washers on top of each spring to preload them.
This will effectivly make them a bit harder to compress. There of course is a limit on how much preload you can use, as you would not want to have the spring compress till it is completely bound before you reached the limit of available fork travel.
If the valve springs are stiffer than the fork springs, you are in essence just adding preload to the stock spring. If they are lighter weight than the stock fork springs, you are not accomplishing anything except adding the comprressed height of the valve spring as a preload.
If your really want to make the forks a bit stiffer, add the washers as outlined above.
Most that do not like the stock rear shocks simply replace them, but there have been a few that have replaced the oil in them. It did not make them any better however...