There is tube, tire parts, air between the braking surfaces in clinchers, not on tubulars. The brake surfaces of clinchers needs more material as they are a 3 corner open box. The same manufactures clincher version will be heavier than the tubular. Seems by 100-200g / wheel - at the rim. I would expect adding a 100g heavier tubular tire like a
PR version would leave you lighter than the clincher equivalent, and be less flat prone.