Yes, modern polyester fabrics are an excellent alternative to natural fibers like duck/down insulation. In fact, one of the major benefits of polyester is its ability to stay warm even while wet. Polyester mid layers are also much more affordable despite being only slightly less insulating. A stellar example of a polyester mid layer with synthetic insulation is the Salomon Outline Primaloft Hoodie; this puffy is one of the most high-performance and cost-effective mid layers in any category. Period.