I have TAMD 74p's in mine and at 1900rpm am cruising at 23 knots. I do know that some of the S 43 had cupped props (including mine) but had them removed due to problems with the shafts so wouldn't recommend going down that route.
I'm afraid I can't help with prop size as I haven't a clue what's...
If you boat is on legs they tend to be a bit more economical than the shaft drives hence the extra tank, however having said that we have no need for leg servicing so I suspect the overall running costs should be fairly similar.
We have 74ps in ours which I believe are the same 480hp as your 75s. Back a few years Volvo used our boat for some test figures which I have a copy of somewhere, I'll dig them out and pm them over, but it topped out at 33knots, with the most economical speed being 2100rpm giving about 26.8...
Congratulations on your choice of boat, we have had ours over 14 years and are as pleased with it today as the day we bought it.
In reply to your question we always run the boat on either the twin tanks or the single tank depending on the length of the trip, if for instance we are headed for...