Her passive healing speed boost doesnt stack with gold backpack, its been tested and shown on stream by people like Shroud (https://www.twitch.tv/videos/381189850 around the 2:07:00 mark). Basically if normal no legends are 100% heal time, her passive reduces that to 75%, but gold backpack reduces to 50%. However the 50% for gold backpack is for all legends so it doesnt actually stack with her passive. So for phoenix it would be 10 seconds normally, 7.5 seconds with lifeline, and 5 seconds for anyone with backpack based on that video.