If one needs to critically decide between the efficacy of in-mast furling vs. slab reefing or in-boom furling-reefing, simply ask the above simple question to your sailmaker..... .
In the case of an 'in the mast furling' main,
the picture is often worse than you paint it.
The furler requires a straight mast to work properly,
so once the sail is set you can not bend the mast
to flatten it. The sail can not be cut with much luff
curve to add draft forward, and the cloth must be
lighter and softer than would be used in 'real' sail
of the same area. Any roachless mainsail tends to
get large hook to windward in the leech when sheeted
All this adds up to a very inefficient sail upwind in
a breeze, with a flat entry and a full back end with the
draft much too far aft. This is why more often than not,
a boat with such a rig is usually motored to any destination
that involves upwind work.