Concrete Radiant floor heating slab thickness?

A double brick walled Church in the Northeast is redoing a basement bathroom. The old concrete was 6" thick. The 7' x 7' (9' high) room has unheated spaces on 5 of 6 sides (under an exterior porch.) Heating was formerly a steam radiator on the ceiling. Radiant floor heating is desirable with a view to install a thermal solar collector. Current information suggests that the insulated concrete slab should be no more than 4" thick with pex tubing 2" below the surface. As the concrete will be acting as a thermal storage mass as well as a radiant surface wouldn't a thicker slab be desirable? How would a thicker slab also be made to be responsive to the changes in temp? Is double layering of the tubing ever done?

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