Look at the mainsheet as if it were different pieces of line with different requirements.

The section that runs throught the blocks, but doesn't reach the cleat, must be very strong, but can be very skinny without a friction cover.

The section that gets runs from the cleat to the skippers hands, upwind, must be fat enough and "grippy" enough to pull and cleat without hurting his hands and yet still run well.

The section that runs from the traveler deadend to the skippers hands must be biggest of all (less purchase), but carries little load. It is hardest to pull because it is only a 2:1 while the main is 7:1 or 8:1, so the load on the line is far less.

The section that connects the "upwind section to the "traveler" section, the "downwind" section, carries the least load of all, but it still must be gripped.

So, if you take a small spectra core line, appropriate to your block selection, you can strip the cover off the section that is behind the mainsheet cleat, pull the core out of the "downwind" section and add the core to the "traveler" section.

You would end up with a continuous line with four different strengths and thicknesses.

It is much easier than it sounds.

P.S.

There is also Marlow Taperable Sheets which has a Spectra core and TWO covers on it. Think of the possibilities :-)