AFAIK the current Thorn Sherpa (if this is what you have) can have two different forks, a lightweight 853 fork that isn't designed to carry any luggage and a heavier fork with bosses for low loaders. I assume you have the latter. This one is rated by Thorn for up to 6 Kg each side, and is the same as the fork fitted to the current Raven Rohloff equipped bikes. Thorn seem to be realistic about the loads they quote, so 6 Kg each side should be quite safe.
When you mention the "raven forks with the steel plate" I assume you mean the twin-plate ones that were fitted to the discontinued Raven Tour model.
I have two Raven Tours with these twin plate forks, and don't notice any untoward flexing when loaded, but I don't know if they are any stiffer than the current forks.
You don't mention the rack you are using. This might be a factor. I have the Thorn low-loader rack, which I find very stable.
If you really want to fit the twin plate forks to your Sherpa, you would be best advised to contact Thorn directly and ask them:
(1) whether it is possible or not. I think fork geometry on Raven and Sherpa bikes depends to some extent on frame size, so mention that.
(2) whether the twin plate forks would make much difference or not.
Personally, I wouldn't bother changing the forks. I'd just adjust the weight carried between front panniers, rear panniers and rear rack top bags till I was satisified with the handling, putting the light stuff at the front if you don't like the feel of the bike with 4 to 6 Kg each side.
