Wheelspin shouldn't even factor in your decision, it's unspring weight - it still has exactly the same amount of car pressing down on the tyres.
Even so, most of the weight saving comes from the centre of the disc being replaced with an aluminium bell, so its rotating mass effect isn't much at all. The caliper will be lighter too, but compared with the benefits, it honestly doesn't matter.
I used Carbotech XP8s on both road and track all year round, they worked from cold and never, ever faded on any track. That was on a 343mm Alcon kit, which a 17" wheel fitted over too. Because the brakes are better cooled and not heatsoaking one big lump of mass, and because you use them less because they're much more powerful, you can use a softer compound pad. Am I right in saying you have XP10s and got them to fade?