A timely thread for me (and I am in the Seattle area too!)
i just talked with a guy from QuietCurtains Quiet Curtains - Your source for acoustic curtains & acoustic drapes. Lab Tested & Field Proven.
The stuff aint cheap but he claims that NRC peak of 1.0 with meaningful absorption down to 160Hz. Like the OP I am just looking at trading an 8 foot outside facing window and 6 foot sliding glass door. For their most sound absorbing stuff looking at like $1200 for an 8 foot by 6 foot section with hardware. Can save $300+ by going one step down and using cheaper hardware. Like I said... not cheap.
If anyone has experience with this company I would be interested wither PM or post....