Excellent choice — #1 of 31 Cashmere ranks #1 of 31 fabrics for warmth without weight. It is one of the best fabrics for this. Ranked on warmth relative to weight, with a packability bonus. It is the top pick.
Cashmere measures 50/100 for breathability, 15% moisture regain, 90/100 warmth, at 200 gsm. Warmth-to-weight is the metric: 90/100 warmth at 200 gsm is an excellent ratio for packable insulation.