Could there be cases where the item has to be screwed on so hard that too much of the putty gets squeezed out? I am looking at screwing down an aluminum plate over all the holes made by a hatch and drip cap I removed.
I can't speak from experience with trailers, but I have done a lot of work on sail and powerboats as an assistant to a friend who has made his living doing it for over thirty years. Butyl tape is amazing stuff - you can stretch it to thin it out, roll it into ropes, double up thicknesses, make doughnuts to seal screw heads - and it stays flexible darn near forever, unlike silicone which goes rubbery. It's also far better for adhesion, and can take far more flexing than cured silicone. I have scraped butyl off boat decks and fittings that was forty years old, and it was as pliable as the day it was installed. I've also seen butyl completely seal copious deck leaks
in a 45' sailboat after we'd removed, cleaned and reseated every single item of deck hardware - perhaps 400-500 holes in all! They'd been seated in some sort of goop that went hard and cracked over the years. (You don't even want to think about what it cost the owner to get that multi-day job done
Re your question - if you have any doubts about it sealing the plate, can you place another strip of tape inside the contact one, use a double-thickness strip, or roll a "rope" that would allow a bit of butyl to squeeze out on either side once the plate is seated? Don't overtighten the fasteners - snug does the job just as well as cranked down hard! Once you see tape starting to squeeze out, that's tight enough. You might want to make little doughnuts of rolled butyl to go under the screwheads. Just trim the exterior excess tape from around the edges of the screws and the plate with a sharp knife, and keep an eye out for leaks
for a few days...
Finally, if you want to apply butyl where silicone has been, make sure it is thoroughly cleaned - we use fine sandpaper, followed by a scrubdown with alcohol, to get the residue off. Any lingering trace of silicone, even an oily film, will really interfere with the tape's adhesion.