Halyard and tackling cleats on a Nacra Inter 20

That is a solvable problem-use a tapered mainsheet. The thick part of the mainsheet only ever runs through the 57 mm blocks in my setup. This is why it is important to learn how to build your own tapered lines, and to use them in high purchase setups.

What is the weight delta between the 57mm and 75mm blockset? I would think that's the issue for racing.

442 g for the 57mm base (sans the 40mm block and tackle)
805 g for the 75mm base (")

340 g 57mm quad top
772 g 75mm quad top

That's definitely on the porky side for a set of blocks. Not something I would consider if you are seriously racing, but a good option for lots of distance sailing.

I don't race at all anymore

I MIGHT do "round the island" this year, but only so i get to crew on a F31R