Maarten,
I'm sure that someone more expert than myself can provide better information on drydown but here's my take: I never try to scientifically estimate change in print tone due to drydown, but instead make test prints with highlights at the density I want and also a little lighter (typically -1/8 stop less light). I then review them after they are completely dry and adjust from there. There are methods for measuring drydown in the book Way Beyond Monochrome, as well as in other sources, but the adjustment seems just as easy to arrive at informally.
40x40 prints (I'm assuming that you mean centimeters - so about 16" square) may have quite a bit of curl, depending in the paper. I use a dry mount press to flatten my prints, but there are other methods. Search on the forums for information on paper curl for techniques.
Fiber takes a significantly longer to develop, fix and wash, which will slow down your process. The appearance of fiber papers also varies greatly from RC in terms of density and contrast so that even if you are going from Ilford RC to Ilford FB, you will most likely have to adjust your times, filtration, dodging and burning, and may not be able to recreate an image exactly as it appears on RC. If you are making a number of prints, this could result in a lot of time in the darkroom.
Fiber paper is significantly more expensive than RC, which can add up if the negatives are hard to print or if you are trying out variations.
Even with all of these issues, I don't use RC paper for anything except proof sheets - the quality of the prints is so much better that it doesn't make sense not to use fiber.