Front end feedback - your advice would be appreciated.

does your weight suit the spring rate ?

you need to work on the comp settings , try going harder , maybe the forks are compressing too much mid turn .

fresh fork oil works wonders .

front tyre choice and pressure should be considered

all of the above imo

going from a 98 fe400 to an 04 fe450 gave me some different steering sensations. the older bike had "slow" raked out forks, and the newer has "quicker" pulled in forks. to make the older one turn, i would have to keep my chest over the bars and try to steer with the rear. with the newer one, in conditions you describe, i find the quick steering can cause the front to slip with little or no warning when i try to ride it like the old one. i have slightly heavier fork springs and also am 2 clicks off the bottom on rebound.
make sure your sag is where it needs to be and play with it by going more or less on the rear to see if it makes any difference. but i'll bet you'll find, as i did, that its prolly the rider that needs adjusted.

what you have described is what a lot of dirt bike magazines skirt over when they say some rides take a while to get used to the berg handling.

It results no so much-in my view- from the steeper rake but the fact that the rake is out there and the wheel base is long and pushes. You have got a better result by increasing rebound, but that will have a negative effect in other handling. The sudden slip described by someone else is because they have too much preload in the fork and when the 550 four stroke power comes on it lightens the fron wheel and with not enough front sag to maintain grip lets go.

The plowing you refer to is caused by the front saging and walling. The solution is to use a heavier spring in the front and not too much pre-load. This will fix your plowing and by not over pre-laoding prevent it letting go as the other thread talked about.

I suggest also what happens is that some find the rear spring too soft for their riding style and jack it up too stop it bottoming. This puts pressure on the soft front spring and causes the plowing. I suggest you follow the correct pre-load setting for pds by making sure you have about 30 -35 mills free sag, but just enough pre-load to drive the front around corners a little. Simply doing that may solve the front problem as I have found berg springs unless over or under the standard weight are the best compremise that have been able to work out.

The other problem may be that you have chosen the 550 because your style is to come into the corner off the throttle pivot and power hard out and this works great on hard surfaces... with the soft surface you are going into the soft turn apex with the power off and the 550 braking affect pushing the front. The solution is to sit in the middle or stand and that makes the soft spring not seem soft, have a standard compression on the fork or heavieer (a lot back of compression as they like it better going over rocks and tree roots) and then .... well power around the soft turn and all will be fine. Of course as you know if you come in and hit the power hard and things are not fine you are going to go down. The 450 does not push like this so you can coast around the apex better and that I think is the real choice between the two bikes. Let's be honest apart from that who would not want the power of the 550... in hard packed goat track in goes great and in the open things are even better, but that soft sand corner push can be scarey.

set the pre-load at 35mm, compression two clicks are harder than standard (for your first run). Sit forward but lean back .... coast around the first time and see what you think.