Excellent choice — #6 of 31 Polyester ranks #6 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. The top pick is Cashmere.
Polyester measures 30/100 for breathability, 0% moisture regain, 50/100 warmth, at 140 gsm. Warmth-to-weight is the metric: 50/100 warmth at 140 gsm is an excellent ratio for packable insulation.